It depends on the stock. If you buy Bell Telephone or the Royal Bank, you can keep it for a long, long time. If you buy a mining venture and don't get out in time, you lose most of your money again. There are cyclical stocks. There are all kinds of stocks.
The same applies with respect to pricing. If real estate gets vastly overpriced in Canada, which in many jurisdictions, especially in the west, it is, I think at that time you might want to take a capital gain, because sooner or later it goes back to the bench line.