Glass vs metal cooking times

I keep reading different views on the cooking times when using glass pans. I am about to make a meatloaf for tonights dinner, and I find that the glass pans clean up so much easier. When I read different recipes for different items sometimes it says to increase cooking times other times it states nothing. Is there a general rule of thumb when cooking in a glass (corning ware or pyrex) dish to follow?

Thank you inadvance for your advice