ok i just did a quick calculation of the whole elite four and gary(with a Venusaur) and i got with 10 vitamins in each stat you would need to beat the eliet four 22 times to max out your stat exp, does that sound about right also very very sorry about my spelling and grammer i know its bad
btw these are the number i had for base stats for all 5 trainners Hp total 1886 Def total 2277 Spd total 2106 Spc total 2090
If in doubt simply solo the Elite 4 and Box the pokemon you're boosting the Exp for and then take it out. If any stats have increased, it needs more training - if none at all increase, then Stat Exp training for that Pokemon has been done.
Note that the stats actually stop increasing when the Stat Exp reaches around the 63,504 mark, rather than the maximum 6,5025 due to the way it's calculated ingame.