A trainer tire can probably last almost indefinitely. I've at least never heard of one failing. Trainer tires have reinforced center tread to withstand the heat from high friction and stiffer sidewalls to maintain good contact with the trainer drum. I know several people who've flatted their road tires on trainers because they neither knew about the need to keep a high pressure nor thought about the consequences it would have on blowing up their inner tubes.
Another benefit of keeping the trainer wheel on the trainer, if it's a smart trainer that can report power, is that one only needs to keep the same tire pressure and not have to recalibrate every ride to get correct power after fiddling around getting the tension dialed properly. For a dumb trainer this isn't important, but it would still be nice to know that you're getting a consistent workout at whatever cadence and gear.