“Five more minutes” is how a whole hour leaves without asking permission.

Each extension is genuinely small, which is what makes it work on you. You are never deciding to lose an hour, only to lose five minutes, and the decision is available again immediately. Time spent that way does not feel spent, which is why the total always arrives as a surprise. Nothing about the individual choice was unreasonable, and that is the trap.

Put the boundary somewhere outside your own judgment. A timer that ends the thing, an alarm set for the real cutoff, a plan made while you are still fresh. Decide once instead of twelve times.