Right now it is something around 96 hours, to help players on holidays etc. to not miss many turns (read the four messages starting from this one: viewtopic.php?f=2&t=11274&start=300#p96790).
But this is not about requirements. I like to play this game, way more against people, and if we all can play faster I'll be happy. If we cannot, I won't be unhappy, it's just a game (and for what really matters these days, playtesting, we are doing great). So please, don't think I expect you to connect more often or follow some rules about frequency.