Yes, it usually happens after 2-3 hours of playing. And no, i don't monitor temperatures.
It probably wouldn't be a bad idea to start. Chronic overheating will kill your graphics card and other PC components. If the computer is a desktop, open the case and check inside for dust build-up; if there's a bunch of dust all over the components, clean them off with some canned air. If the overheating continues, you'll need to check the fans for proper operation and replace any that aren't working, and maybe even add a chassis fan to improve airflow. If it's a laptop, check the air vents for dust build-up and use canned air to clean them out the best you can. You can also get a cooling pad to help get rid of heat.