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.