How long can you grind?

I only grind when there's a big goal in front of me. On my Hermit's 60 - 70 I grinded...7 hours straight to get from 69 to 70, maybe more than 7. Spent 8 hours getting 1 - 32 on my Sniper in one day, pure training. During Hybe's 60 - 70, I spent the whole day getting 3 levels to end up at 68.

Nowadays I barely train at all; longest I've trained was 4 hours during a 2x event.