If you want baby potatoes like for canning or stews, usually after about 75 days you can harvest them. I usually wait until the tops start dying off if I want bigger potatoes to store for later. Also if you see potatoes "heaving" (exposed to the sun) you need to cover them back up with dirt. Just do a test dig. If they aren't big enough you can cover them back up and leave them for a couple of weeks.
Did you plant them before St. Patricks day?

__________________
I am here for only a short time on this earth. My goal is to make everyone I see smile if only for a moment.