I give them a time frame that I will work within (usually a month) and don't ask to be bothered during that time. if i think i am going over the month, i will contact them for that update.

otherwise, i contact them in sketch stage, and if i have any questions. I wouldn't contact them more than twice for different stages--once for rough sketch, once for maybe flatted colors if you're worried? anything more than that, unless you're asking questions, seems redundant.