At the very end of a book I can manage to work for longer stretches but mostly making stuff up for three hours that's enough. I can't do any more. At the end of the day I might tinker with my morning's work and maybe write some again. But I think three hours is fine.
There is also this benefit in brag that the speaker is unconsciously expressing his own ideal. Humor him by all means draw it all out and hold him to it.
