You can safely set the fan speed to anything you want above stock. I keep my GTX260 at 75% instead of 100% as to keep the noise down some, and it stays 35-38C idle, others keep theirs at 100% and it's safe. Even if the Fan goes out due to being at 100% so long, would take a long time anyways, it's easier to replace a fan/cooler then an entire GPU.
Another thing to think about is it's doubtful the card is reaching max temps, unless you have a hot case temp. Most cards can handle up to about 85-100C, although no one wouldn't recommend it.
So, 50-75% is a good speed.