From the way you worded your question, it sounds like you are talking about how long you should hold a stock that is down before selling it, and that will generally depend on why you are holding it, in what account (i.e. for long-term purposes or short-term goals), why it is down, what else you would invest the money in, etc.
Many people will set a target where if a stock (or fund or ETF, etc.) declines a certain percentage from where they bought (for example, 10%), they will automatically sell and move on to something else. However, that is not always the best bet.
If you have done your research and the company's fundamentals are sound, then it may not be wise to sell just because it goes down a certain percentage. For example, is the entire market going down (like in 2008)? Is the stock's sector being hit hard for some reason and it is being lumped in that way? Is there just a general dislike for the stock at the moment?
As long as the company remains in a good position, it may be worth holding onto it even when down. Often, stocks will be hit for some reason that may have absolutely nothing to do with the quality of the company and have everything to do with emotions or heard mentality leading to a decline in price. In these cases, the stock may rebound and you will be better off in the long-run. However, sometimes a stock goes down for a legitimate reason, which may make it best to sell and move on.
Remember, that when you hold a stock that is down, you only have paper losses (i.e. the stock may recover to where you will be back to even or see a profit), but when you sell, you lock in those loses.
At the end of the day, there is no right answer, as it will vary for everyone. You have to assess whether you believe the company is still a good company, why you bought the stock in the first place, what you would purchase with the proceeds from selling it, etc.
Best of luck.