Minimum fpga dev-kit/fpga

FPGAs support a limited number of reconfigurations, but this number is high-enough that they can last multiple years of intensive use. Other than that, I am not very sure about the effect of time on an FPGA. I think you would be able to get a more accurate answer from the more experienced people in the "FPGA, Hardcopy, and CPLD Discussion" section. Note that if your aim is to use OpenCL, make sure that the board you are buying supports OpenCL.