Since most investors fail to evaluate a company before buying it, they usually have no preconceived idea of what they can earn over time.
That is a fatal mistake.
You should be able to estimate the annualized rate of return you expect from every investment and be able to quantify how you derived that figure.
If your criteria for investing are vague or ill-defined, you are more likely to make mistakes about selling.
Are you looking for a 50% price increase in one year. If so, what factors can cause that to occur, and how likely are those factors to occur?
Are you looking for 15% a year for 10 years? If so, calculate what the stock must trade for in 10 years and determine the earnings that will be needed to support the price.