When to exit from any MF?

[Policy_expert]

This seem to be a tricky question for me. If I want to exit from a MF, when should I exit?
If I am sitting on a profit then what is the best possible time to exit?
After certain target % acheived? Or after certain time frame? or what?
Your views?

I would prefer exit after % target is acheived. We invest for target.