GPU fans are likely to fail very fast if you keep it on 100%, the bearings will wear out in at most 2 years.
A mechanical engineer's perspective on this... it mostly depends on the bearing quality and number of cycles. If your steel is flawless, they will last a long long time no matter how fast you run them. Otherwise, if there's a flaw (in real life, there always is), it comes down to how big the flaw is and how many cycles you run. Tiny cracks will get bigger over time. I think it's a linear relationship with # of cycles.
Anyway, the fan should last twice as long at 50% speed as 100%, if the steel is high-quality.http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/anal/kelly/fatigue.html
The important equation is about halfway down