Fortnite can pull time through matches, quests, friends, events, and the feeling that the next round will be better. The stopping rule has to exist before you queue.
Choose your limit: one match, three matches, or one planned time block. Add Fortnite to Apple Screen Time and track sessions with Sandflow so the timer stays visible.
If you play with friends, say the limit before starting. It is easier to leave after the planned match when the boundary was clear from the beginning.