I would say this:
1. Don't double post. (There'd be no point anyway)
2. If another player is interacting with you, you can post in direct response to them back and forth, but try not to go too crazy unless you're positive no one else wants to jump in as well.
3. If your character is on their own and no one has directly done anything in response to your last post, once say two or three other people have posted, it's probably safe to post again and move things along for your character. Or you can continue to wait for a good opportunity to do something. It's a judgement call at that point.