using repetition efficiently, building reliability and consistency

[alice1233]

When a very short passage (no more than one or two measures) doesn't feel reliable, how long should you actually practice it? I know techniques such as rhythmic variation and other approaches, but I'm less interested in what they are than in how to apply them effectively in practice.
Since piano playing is learned largely through repetition, I've been wondering how much repetition is truly useful. Of course, endlessly repeating a passage isn't productive, and there's no exact number of repetitions that guarantees success, but do you have a general rule of thumb?

Sometimes a passage seems secure after only a few successful attempts, yet it still fails unpredictably later - a few days afterward, when performing, or even just minutes later. Other passages never seem completely dependable and work only, say, eight out of ten times. How do you practice sections like this in a way that creates genuine reliability rather than only temporary success?

[brogers70]

I don't know if there's a reliable rule for that. What I try is to isolate the exact sport where something goes wrong and play just the few beats around that spot three or four times very slowly. Then work on something else. Then come back and do the same thing three or four times. Work on something else. Come back and add a measure or two before and/or after the trouble spot. Do that longer bit three or four times slowly. Then put it aside for the day. I do that for a few days, maybe getting the tempo up to the real tempo, and then integrate it into the piece. If I find that after a few weeks that same spot needs tuning up again I just go back and do what I did before. Over time it solidifies for me.