Write down your exit condition before you start

High value
Evidence B
Saves time
Free
Takes minutes
Some willpower

In plain words

Before you start, write one sentence: if you haven't hit a certain target by a certain date, you stop. Pooling several studies, writing it this way does help people pull out of things that are doomed to fail. What you save is all the time you would otherwise have kept pouring in.

Cost: Free. Spend 5 minutes when you start writing down "if I haven't reached a certain target by a certain date, I stop." The hard part is admitting it when the day comes. Stopping in front of other people can feel a bit embarrassing.
What you get: Pooling several studies on implementation intentions, the finding is that an "if... then..." decided in advance also helps people pull out of things that are doomed to fail. The original wording is "disengagement from failing courses of action." The time saved is whatever you would otherwise have gone on investing.
Notes: Set the exit condition right when you start, while you are calm. If you set it later, the cost you have already put in will bias you. When you judge, look only at future effort and future return; see §4.3. Also, "being willing to pull out" is only one sub-result in the pooled studies, and no one has calculated on its own how much time it saves.
Evidence grade: B : Supported by research but without a precise number, or based on small or single studies.
Sources: Gollwitzer, P. M., & Sheeran, P. (2006). Implementation intentions and goal achievement: A meta-analysis of effects and processes. Advances in Experimental Social Psychology, 38, 69–119. https://doi.org/10.1016/S0065-2601(06)38002-1

Translated from the Chinese original. Original book, section 4, tip 2. Content may be out of date and rules differ by place. When you actually deal with an agency, a doctor or a court, follow what local officials, doctors and lawyers tell you. Disclaimer