I don't know much about FFT multiplication so someone please correct me if this is wrong. Assuming the underlying multiplication is of the same order of complexity for base 5 as for base 2 then the time to prp a base 5 number should just be a constant multiple of the time to prp a base 2 number of the same size.
(By same size I mean one with the same number of digits. b^{n} has about n*log(b)/log(10) decimal digits.)
If we gan get a few times for some base 2 and base 5 numbers of the same size then we can work out the multiple.
