How long can you grind?

Nowadays, I can barely grind at all unless there's a 2x event going on. In that case, I'd buy a 2x card and train the whole 4 hours. Regular training? pineapple that; I wouldn't last more than two hours. The most I can ever recall grinding was during that one event that was on... Easter? I don't remember. But anyways, it was a 12-hour 2x event, and me and my friend kept the map and trained on 4x the whole time. I went from like, 112 to 116, lmao.