Every 46 minutes, there is a 20% chance for a raid to spawn if all conditions are fulfilled.

It's quite possible to have raids every other day for a while, and then no raids for a long time.

If you really want to make your base troll-proof, either dig a trench around it or raise an earth wall around it.

You will want the trench to be at least 5 wood floor pieces away from your structures to ensure trolls cannot toss pebbles at it, unless the trench is deep enough that you can't see the giant blueberry's head.

If you don't want to deal with terrain alteration, build a series of walls around your base. It doesn't have to be a complete wall, and in fact an incomplete wall may work better. The enemy will try to make it's way to you, and if he can't get to you, he'll start smashing stuff. If he CAN get to you, but he needs to take the long way around, you can harass him until the raid ends.