Does your xbox 360 get hot and if so is it normal?

i just noticed my xbox is getting really hot in the back and so are the discs in my tray..i was wondering if that is normal and everyone i have spoken to says its normal...im just worried about blowing it out and not having one...also i usually play for a couple hours then let it rest..and its in a cool ventilate area...when i first turn it on what is the usually time it starts getting hot?