How long is too long to play in one sitting without my laptop over heating?

I have a gaming laptop that is brand new and it's a Lenovo y410p. I've been playing for four hours on World of Warcraft with my brother and his friend. We plan on pulling an all nighter. My brother just moved so I barely get to talk to him. So this is a once and a blue moon opportunity. I have a graphics card in my computer and it gets pretty hot on that side. The computer has good ventilation and it is on a desk right now. Will it be really bad for my computer if I play the whole night?

Please answer.

I would be more concerned for you not the computer. The computer will be fine but if you feel it getting to hot then just lift it up and let it breath for 3-5 minutes. Looking at a computer screen all night is not good for your eyes so I recommend taking a 10 minuet break every 1-2 hours just so that you don't over heat. Keep yourself hydrated when gaming too.

Hope I helped!

Well, it might be fine to do it for once, however, I suggest that, for future reference, you get yourself one of these cooling pads that you can connect to the laptop and they will keep it from overheating… I have one, and my laptop keeps going for hours and hours without any overheating…

Mohammed's idea is very good, about getting a cooling pad. They have fans to help cool it more efficiently.
But it you want to go low tech or cheap, then just put a little book under the back of laptop, to elevate it a little bit, so that air can flow underneath it. I do that and it helps reduce heat quite a bit.
A low or thin book works better, because then the laptop isn't tilted so much as to make it hard to type on it.