So how long can you grind without getting bored, tired or sleepy
It depends on how you define 'bored/tired/sleepy', I grind MTTs and I love my 5-6 mins breaks, I usually have 1-2 coffees and I consider this as a supplement to keep myself in top condition... over the years, I had some really long sessions (like 15+ hours) and I'm still relatively young with a pretty active life (I play different sports weekly * mostly football - sometimes even daily), so I can play poker and keep a high level of play, close to my A game, for 5+ hours... still, even tho I take poker seriously, I still consider myself a recreational player and poker is mostly a 'hobby', with family and work always a priority - so my poker sessions are getting shorter and I even consider switching back to mostly cash games as it will allow me to manage my sessions much better and/or stop playing when I notice a drop in decision making and/or something IRL comes up.