I think that doing 3, maybe 4 RPs at once is optimal, but I rarely follow that rule. A lot of it depends on how active the RPs are. You can be in quick RPs that have shorter posts, a couple paragraphs. Or you could be in one where posting ten paragraphs is an average post.
In general I think it highly depends on the person, and how much time they want to commit. Those who post longer posts might want to invest one chunk of time once or twice a week, while those with shorter posts might only use 20-30 minutes once, maybe twice a day.