Originally Posted by fumje

I believe the answer is always, as they rotate through. It's not a "just-in-case pilot" seat, it's to make sure that none of the pilots working the flight has to be "on" for the entire duration (and potentially fatigued / lose attention).

I'm not certain what the cutoff for needing the rest seat, but from what I've noticed it's about 7 hours flying time. Note that 7 hours flying time is a fair bit more in working time, with preflight checks and taxi.