A team of assistants! really the concept of mixing 3-4 songs a day isn't happening..maybe that's true if your sessions are so prepped you only have to tweak final balances, but a mix a day is more realistic (maybe a mix and some recalls).

Personally, having a young family, given that I'm up at 6.30ish but don't start work til 10.30, I find by 6-7 I've had enough - esp if I'm mixing on my own! I still usually get to the "1st rev" of a mix in that time, but I do find things take me a little longer. And I don't work weekends unless I absolutely have to! but that's ok, things speed up and slow down according to your time of life.

I find I struggle to find time to actually play music anymore...that's what gives in my lifestyle! Which I'm trying to rectify, but it is hard.