You can only get 255 in one stat, so it's no big deal if you over train. For example, if you faint 261 attack EVs worth of Lillipups and Patrats, it only counts for 255 and the other 255 won't be wasted. Hope I helped :) Source: Experience.
P.S. If you would give me the stats, IVs and level of the Pokemon you are training, I can figure out their EVs using a simulator for you.
You can figure out the IVs Here.