The wait time varies alot. I know last fall authors were waiting through three updates before their stuff was posted, though I think the backlog has been reduced by now. And updates are done sporadically, rather than regularly.
Don't worry about being accepted, though. As long as you meet the minimum length (10k, and sometimes even when you don't) and don't write about a banned topic (e-mail Jinn for details), he won't turn you down. His policy is that the market (readers and reviewers) should decide, not the gatekeeper.
Good luck!