0 seconds. Anything else is idiotic.
There is currently a block subsidy of 25 BTC. Ever second you aren't hashing for that is a significant reduction in gross revenue. Now maybe someday when the block subsidy is much much lower and fees are much much higher (say in 32 years = 8 block subsidy halvings) it may make sense for some miners to go offline but today the math is simple ... you wait 0 seconds.
So what you're saying is, miners should disregard any increase in potential payout? At what point exactly does the increase in payout become relevant, and can you prove it is irrelevant now?