At least once a week would be preferable, if you can manage to get your character on their own then you can take longer between posts because you're not slowing anyone else down, so to speak. 

How much you write is up to you, I guess I'll put the limit at 2 paragraphs minimum but if you plan on writing only two paragraphs at least take some time to make 'em good.

This is still a work in progress too, I'm currently writing about the 8 families of Siroth and will then make a character sign up and do bits and bobs to improve the story. Planning on just chucking up an OOC hopefully the finished product will attract more attention...