If you are using a water cooled (or air cooled) gas filled chamber based ionising laser, then yes, they can overheat if something goes wrong in the cooling.
If you are using a pointer then unless something is badly wrong with the unit in question, NO. they will not.
thanks. the reason i wanted to know is because i want to strap one to my pellet gun. it has no recoil so it wont hurt the inner workings of the dpss laser.
The thing with greens is they have an infrared laser diode in them that is putting out much more than 5mW, but still it isn't putting enough oput to make serious heat. If you feel the laser getting warm I would shut it off.
most 5mW greens i've seen do have a recommended duty cycle. the laserglow galileo, however, apparently can be left on indefinitely (and has a push-on, push-off button, instead of hold-for-on).