What is a good exit strategy for investing? After what percentage increase or decrease should you sell your stocks and why?

Investing is not that easy. When to buy and sell stocks does not follow a formula. You have to be knowledgeable about each of your stocks. Is a dramatic rise in a stock an over optimistic reaction of investors or do its profits and future profits warrant the price increase? Is a decline likely to be temporary or is the future outlook for the company bad?