like thndr said, 100yds is the standard...
shorter than that, and you cant read them properly...
farther than that, shooter error, wind, etc etc becomes magnified...
i supposed, ideally, the farther the better as your groups are magnified and easier to read and see what loads are consistent...but too many variables for most guys to deal with IMO...
when testing loads recently i fired a 3 shot 1 ragged hole group at like 35yds (was getting the scope on paper..) that same load at 100yds was one of my largest out of 7 or 8 loads...