Depends on what you're cooking. If it's meat, you need to be sure it comes up to the proper temp to kill the bacteria - so stick a thermometer in it and keep checking every 10 or 15 minutes. As the PP mentioned, you can cover it with foil to keep it from drying out - this will also keep the heat circulating inside the foll to cook the meat.
If it's a casserole, like lasagna or something else pasta based, just cover it with foil and check on it every so often - take it out when it's hot and bubbly.
at 9:58 AM on Dec. 24, 2009