How long for will GPU still be worth it?

[simplydt]

Let's put the cost of each GPU aside for a second, those can always be re-used or resold for gamers. So assuming investment cost is $0 and electricty cost is cheap ($.10 per KW), then would running a PC 24/7 in a pool, with 4 graphic cards on the PC, be worth it still?

How long for do you reckon this will give any ROI for?

Thanks for any help
-D