Also trying to make this calculation now; I'm currently running one and participating in four others. So far that seems manageable, but the more correct answer seems to be that It Depends--on how quickly you write, how much you write, and how often your fellow participants are posting. 

So far only the campaign I'm DMing has really even given me an opportunity to post more than once every few days, and I think that's mostly because I'm used to a more rapid-fire, in person style of DMing.