It can vary by the airlines approved program. I know Delta's programs vary by aircraft type. I work on the MD88 HMV line. Our 88's fly for six years between heavy checks. They only get a overnight visits for basicly lube jobs between heavy checks. The heavy check lasts about a month, and no the plane does not get stripped of paint. It is a very detailed inspection of the airplane though. All areas get looked at for corrosion and structural defects. It is also a good time to do any extensive mods on the plane, like adding a hud system or changing the interior.
Other types, like the 737-800 only get PSV checks. These checks are only a few days long and spread the heavy check items out over the same six years. Our 767/757 fleet get a hybrid of the two.
Go over to the Tech/Ops board and do a search. This topic has been covered over there in a lot more depth.