How often do you have to replace he battery?
I replace mine (on average) once a year. If I see activity is not being detected then I will replace the batteries. Their RF transmit range will drop off before the batteries drain completely.
Once programmed do they tend to be trouble free or do you have to tinker with them?
They are very reliable. You won't have to touch them until you need to replace the batteries. Inside a condo you won't have any issues with rain (hopefully). The only issue I ever had was the off delay adjustment rubber cap drying out in the sun then allowing water to seep in during the rain. Had one unit behave odd due to this. All my outside ones now have caulking over this cap to ensure a water tight seal.
I am going to try to operate things inside my condo by detecting the presence of people in the different areas and since all the walls are finished I have to use wireless transmitters. Is there much of a problem with interference? TV's, walls with metal studs, neighbors electronice, fluorescent lights, etc.
There shouldn't be any electrical interference to worry about, but metal objects between it and the receiver will decrease the range.
How far away from the receiver can it be mounted?
The answer to this one depends on what receiver you are (or will be) using. With a good receiver and no obstructions they have a good household range. I have never actually measured their range with my V572RF32, but my furthest away is ~60' with a metal garage door in the way and never misses.
