Hi
It's 30 regular days before expiry of your current status. You will be on implied status when your current status ends and you are waiting for approval of the extension. So if it expires May 30th, apply to extend May 1st; after expiry on May 30th, you are on implied status until you get a decision from CIC about the extension.
Basically, you can apply as many times as you want to extend. It's just a matter of proving to CIC that you can continue to support yourself and are still a temporary visitor and don't plan to stay in Canada forever. There are a few people on the forums that have extended their visitor status 2 or 3 times but I imagine if it gets to any more than that, chances of being approved will drop.