It sometimes takes the polyps a while to get "settled" in, but once they start to split, they can do so at an extremely rapid pace. One shroom can turn to two in a month, and two to four, four to eight, etc. I actually find them pests, albeit pretty pests
Depending on the conditions they might sit around and do nothing for a long time and then one day they go berserk and take over. Other times they start out in spreading mode and slow down over time. But usually they can spread fairly fast when they want to.
You can also split them into smaller pieces quite easily and increase your colonies that way. Here's a really good article with pictures on how to do it.