How long can a Motherboard last without any Standoffs

Solution
You should never install a motherboard into a case without using appropriate stand-offs. You could very well short out your board.

-Wolf sends
If there are six holes in the m/b, you're fine. If there are more - get additional standoffs and put them in. The choice is yours - couple bucks for standoffs or potentially 150 bucks (or more) for a new m/b, maybe cpu, maybe gpu, etc.
 
YOU CAN DO IT!! DO IT NOW!! Visualize that in my Arnold voice, lol. Go ahead and try it we'll be happy to sell you a new motherboard, CPU, RAM, and VGA card, and what ever else you burn out in your case from your experiment. Be sure you get a stop-watch to see how long it all lasts.