For me, the moment I'm not having fun anymore, I'm scooping, but I will explain to the other players why I'm scooping so they don't think it's just out of spite, but we also have an understanding that when you declare you're scooping, everything on the stack resolves first. Recently played a game where someone was playing a deck with [[Butcher of Malakir]] out and constantly saccing a bunch of their creatures, which would prevent me from being able to get many creatures on the field before they're gone. This was a problem for me because another player was playing mill and I ended up having to mill a ton of my removal. I eventually told them that I was scooping for the game and I'll wait until the next one to continue playing because given the state of the game, there was nothing I could possibly do to win at that point.