I don't think they have a limit over their lifetime. I think it's more that when their end draws near, their power diminishes.
Q was near his end, but he had marshaled enough power to return four people to 2401, though that effort would terminate his existence (as a Q anyway). When Rios chose to stay behind, Q was able to divert one-quarter of that power into saving Elnor.
Good points!
But according to Quinn on Voyager, it seems that it's possible that the Q Continuum isn't as powerful as they pretend to be. A fixed amount of energy they have for their shenanigans (especially if they are based on technology) would fit with what he said.