as long as you wan would be your answer
until 2 days ago i had a 5 years old laptop a dell inspiron 8600
cpu 1.7 ghz (single core)
1.5 gb -> 2gb
ati radeon 9600 (128 mb)
i could run the game 24 h per days as long as i wanned befor overheatting
the game had a bit of spike lagging when new object apeared on the screen befor i switched to 2 GB but at 2 gb the graphic quality was at max settings and i could follow a fighter in a battle with 206 worlds in a map
my laptop overheated one time and it was when i was playing supremme commander and there was so much humidity that day that i could swim in my own bedroom and the laptop just made him self an emergency shutdown with no dmg
however this depend on your definition of overheating the laptop had the bad tendence to get to hot to be a lap-top but the systeme or performaces never suffered the heating so i could play as much as i wanned as long as i could find a surface to put it on
so as of what my experience can tell me keep away from compac and toshiba for gaming as far as i saw they use to integrate the gpu to thier mother board even ati card on most toshiba are integrated to the motherboard
for hp flip a coins and cheq the phases of the moon to know if it's a good day to buy a hp
for dell the more you pay the more likely you are to have a good systeme (keep away from the new inspiron 1525 the one i tested allmost died when i putted sins on it)
keep away from ati when buying a desktop computer you'll get wierd warning about your psu and the drivers installation will be a pain in the ***
but keep away from nvidia when buying a laptop the chip never give anyting out standing and will be out dated in a year or so (added to the overheating probleme i obviously never faced after all others i saw)
and last but most important if you are not readdy to spend for an XPS, alienware or any equivalent monster systeme from other brand you should keep away from laptop gaming