1st and 2nd gens were 6 year cycles. 3rd and 4th were each 7 years. I'd put money on at least 7 years for 5th gen given the low volumes.
In related news, here are other lifespans for Toyota BOF....
FJ Cruiser - 8 years (only one gen)
Sequoia - 1st gen 6 years; 2nd gen 7 years (through MY 14 - and talks of discontinuing after 2014 MY, but I haven't seen anything recently on this)
Tacoma - 1st gen 10 years; 2nd gen 11 years (will be same gen through at least MY 15)
Tundra - 1st gen 9 years; 2nd gen 8 years (through MY 14, and will continue)