How often do I roll power recharge rolls and at which point in a turn do I re-roll them?
I'm trying to find the official definition of the recharge mechanic in the online compendium and I'm not seeing it there.
You Roll Once per Turn, at the Start of the Monster's Turn
Monster Manual glossary explains recharge:
You would roll it whenever you resolve the "start of turn" phase of each monster, at the same time, for example, as it takes ongoing damage. You roll once for each power that has been used; if the monster has not used the power, you do not roll to recharge it. You can't "bank" multiple charges.
EDIT: I should note that in more recent publications (eg: Rules Compendium) the explanation was change from "start of the monster's turn" to "start of each of the monster's turns", clarifying that you do roll to recharge each turn for a monster that takes multiple turns in a round.