1 time per year
In the off-season when I'm bored , I check all my packs and if they need balanced I balance them perfectly. 50% are already balanced from the year before.
I use many different multiple packs together in series so it is very important that all cells are balanced (even within different packs) and once in a while I will plug one of those lipo checkers onto a pair of packs, but trouble is they are not very accurate.