I reckon I'm luck if mine last more than three or four months.
They're not very robust, though some brands seem better than others - e.g. Sandisk seem better
Usual problem I find is that the metal USB plug gets twisted at an angle and breaks contact with the circuit board, bending it back sometimes works
At present I tend to use ones where the usb plug is plastic not metal, and moulded as an integral part of the design - Sandisk do a "Cruzer Blade" which seems quite tough