I practice the shot until I have it mastered.
The way to practice a missed shot that I should not have missed is first to determine why I missed. If it turns out that I missed because I did not know how to play the shot then I take it to the practice table.
At the practice table I want to learn how to shoot the shot and all its variations. This means that I start out with a simplified version of the shot. Probably I would begin with using center ball on the cue ball and no position. My intent would be to make it three or more times and get to the point where I felt like I had mastered the shot anytime I set it up. It is now one of those shots I own.
The next phase, for me, would be using all the variations of spin on the cue ball that would be needed in various situations. Following this I would practice all the positional variations such as leaving the cue ball in the middle of the table, on the opposite rail, at the end of the table, etc.
When I practice my goal is to make the shot at least 90% of the time. There are not many regular shots that I can’t make and this is not a difficult goal to reach.
I also know that when my shot making drops much below 80% that I need a break. My eyes need a rest, my concentration wanders, I lose interest. There are many factors and I know that if I continue to play I will only develop bad habits. To improve my ability it is best to go and get a cup of coffee, talk to someone, check the net, whatever, for ten minutes or so.
When I am refreshed I come back to the table for another session. To give it everything I have when playing pool it requires a lot of concentration and real effort. After about 30 – 40 minutes of intense effort I need a break.
There are times I have played 14.1 continuous for more than 12 hours with a friend. It is not that I can’t play; it is that I can’t play my best and learn better techniques with the required intensity for more than about 30 – 40 minutes.
How many times should I practice a shot? Heck, I don’t know. I practice it till I own it.

There is one shot I have been working on for about three months and I still can’t quite get it where I want it.