In the summer, I think you can get about 10-15 hours of OK play. In the winter, maybe 15-20 hours. I use Signum Pro polys and they retain tension better than most. I string a touch higher 50-52 poly mains with multi cross +3-4 lbs above main tension.
If you hit the ball really hard and play hard hitters, the numbers will go down. I play around 4.0-4.5 level and get roughly the hours quoted above.