The Answer

Beware of simplistic answers to this question. There are five main factors that will determine the range of your signal.

1) height of antenna, in relation to local terrain
2) location of receiver
3) receiver quality
4) transmitter power
5) RF environment

1) Height of antenna, in relation to local terrain:

FM signals travel in line-of-sight, which means that the higher the antenna is off the ground, the further the signal will travel. Physical obstruction in the way of an FM wave will tend to stop the signal. Imagine, instead of an antenna, that you have a hundred watt lightbulb. If the bulb is lit inside your bedroom, only people in your room will see the light, along with perhaps someone peeping in your window. If you put it on a 20 foot pole on your roof, your neighbors will all see it, as will anyone who does not have any trees in the way, but people living behind the nearest ridge will not see it (that could be a ways away if the terrain is pretty flat). . Now imagine that you put the bulb up on a hundred foot tower behind your house, or on top of a nearby watertower or apartment building. People will probably see that bulb for many miles around.

2)Location of receiver

In fact radio waves are the same thing as lightwaves, only at a much lower frequency, and with a longer wavelength. These waves behave very similarly- just like light, when they meet an obstruction, they can either go through it, be refelected off of it, or be absorbed by the surface and re-radiated as heat. The main difference between ordinary light and radio waves is that ordinary light can only go through "transparent" objects- many more sorts of objects are at least somewhat more transparent to radio.

In diminishing order of "transparency" to radio, here are some things that are common building materials:
Glass
Wood
Masonry
Metal.

All other things being equal, someone listening in a woodframed single familydwelling may be able to hear you ten miles away, while someone in a mortar and steel apartment building with small windows might not hear you five miles away. You can count on them to bounce around a few times, but the further you get from the source, the weaker they get. When I operated a pirate station, Radio Mutiny, at twenty watts, we would frequently get calls from People driving around eight to ten miles away. One guy regularly listened to us from 17 miles away, in New Jersey using a rooftop antenna and a high quality receiver. On the other hand, some people who were three miles away had trouble tuning us in on clock radios if they were on the wrong side of the house. This brings us to the next factorÅ 

3) Receiver quality

You have probably never thought about it too much, but FM receivers vary greatly in quality. Most of the cheaper walkmans and clock radios, especially those with dials, can only bring in a few of the most powerful local stations. On the other hand, most car stereos have very good tuners that are capable of performing well under challenging conditions- moving from one stations territory to the other with a minimum of static. You can test this by taking a cheap boombox in a car with you and compare the performance of the two radios- you will probably notice a dramatic difference in the number of stations you get and their clarity. Component stereos, with antennas (especially those $20-$30 active antennas) are also generally excellent, and can bring in far away signals with clean reception. Digital tuners are usually, but not always better. Somewhere in the middle are boomboxes.

4)Perhaps the least important factor is power.

That is lucky for you since you've only got a hundred watts while the big commercial stations are allowed 20,000, 50,000 and more watts. The light bulb on the pole metaphor is helpful here again. Obviously, your neighbors will see a fifty thousand watt light bulb on your roof better than the hundred watt lightbulb. And the sheer power of the light will be so much that even a neighbor with a window on the opposite side of the house will notice the refelected light from your giant bulb, even though there is no direct line of sight to the bulb. This is somewhat different than the original case of the hundred watt bulb where the reflections will be weak and not nearly so obtrusive. Power definitely helps, it gives you some oomph to punch though walls and bounce a bunch of times before fading out. But even with the fifty thousand watt signal, your giant light bulb will not be able to be seen through the nearby ridge. Depending on terrain, your hundred watt lightbulb on the hundred foot pole may be seen further away then the fifty thousand watt bulb at ground level.

If you look at the FCC channel finder at the map of projected coverage for your station, it is important to remember that these maps are very approximate in nature. The program will draw a circle that is exactly 5.6 kilometers (3.5 miles) in radius around that point. This is the radius within which just about any radio receiver, in any reception condition, should be able to pick up your station. Some receivers (especially the radios in cars, home stereo systems, and radios with external antennas) will pick the station up to two, three or four times further than this "predicted coverage." Some (like Walkmans, and some clock radios) may not even pick you up at the outer edges of your "predicted coverage." There is no brick wall that stops reception at 5.6 kilometers away. It is best to imagine FM coverage as a sunburst, with the best, densest coverage closest to the transmitter and steadily diminishing coverage as you move away, with shadows behind hills and obstructions. Fortunately, your FM radio does not get quieter as you move away from the transmitter: instead the quality slowly degrades until another signal from somewhere else overpowers it. Which brings us to the last factor, RF environment.

5) RF Environment

RF is short for radio Frequency radiation. If there are no other radio stations within a thousand miles on your channel, you will be heard much further away then if every person on your block has their own radio station. Also, if there are no radio stations nearby on adjacent channels (e.g. you are on 91.3, and they are on 91.5, you will also have better coverage. The FCC predicts your coverge based upon the fairly dense signal concentration in the big cities- if you are in a relatively rural area with few radio stations, you will probably go much further than the 5.6 km predicted radius. It is helpful to check your channel for opposing distant stations. If your channel has been approved by the FCC channel finder, it generally means that you are VERY far away from these stations, probably at least 60- 120 kilometers, and probably more. You will probably be able to blow over them in your area, but as you move further towards their transmitter, they will blow over you. Full power radio stations have protected radiuses where their coverage is guaranteed. Anything outside that radius (which is very large, by the way) is considered to be a bonus, and is not legally protected in any way. Low power stations do not have protection from interference, except from new translators and other low power stations. You will have to accept any interference that is caused to you by full power stations, unless they are cheating and using more power than they are supposed to.