A couple of things I've observed over the years about FRS radios:
If you look at the FCC type certifications for FRS radios, while they are advertised as "500 milliwatts", the actual ERP (Effective Radiated Power, which accounts for antenna gain, etc) is often well below 500mw, often more like 200 - 300 mw. The limiting factor is the antenna. This is by design, FCC intended FRS to be a short range service, so the rules about not being able to remove the antenna are there to keep you from adding a higher gain antenna.
So, 300mw into an antenna designed to be lousy at best will result in short range.
Poor receiver sensitivity, due to mass production, component value variances, etc can often make them a little hard of hearing.
UHF is pretty good at building penetration, not so good at foliage/tree penetration. How far they will reach will often depend more on your environment than anything else.
UHF is more of a "line of site" band. The higher up the antenna is, the farther it will reach. Urban ground level distances could be a mile or less. Get one up on top of a building or mountain, and you will see much farther distances.
Short answer, I wouldn't count on more than 1/2 a mile. Less if you are trying to use the radio inside a car (basically a big steel cage that blocks RF).