I don't think there's a set time but I try to be consistent when it comes to publishing. Currently I try to publish 2 hubs a week or a hub every 3-4 days on average. They say being consistent helps but understandably not everyone has the time to write weekly hubs because of other responsibilities.
I also know some hubbers have published a lot of hubs and then let them sit while they moved on to other things. They've essentially gotten the ball rolling so to speak and moved to other tasks.