The voting for charity should be done before the funds are collected for that month.  I'd hate for you to turn people off by using their coins for a cause they do not want to support.  Think about it.  Losing the vote for which charity receives the funds before funds have been secured would only make you be more active in the community to win the next months charity of choice instead of feeling robbed and quitting the community.

Thinking it through I think the time frame should be a week at a time to increase community involvement.