question is, what effect you want?
people often "overkeep" plants, because they want be sure, but honestly, i like to take them bit earlier, because i dont like the couch-lock effect.
what kind of high you want?
if you more energetic and "doing things" - like high, harvest earlier, if you want the the couchlock-kind of high, wait more.
and one more thing: you dont have to harvest everything in one time, so you can have different kind of high even from same plant.
and if you harvests first the main cola, lower ones will grow better if you giving some time for them.