A lot of long term investors follow Warren Buffet. Buffet has probably done more to make value investing popular than any other man. His favourite holding period is "forever" and he has been amazingly consistent and held on to his old investments like American Express, Coca-Cola, Washington Post. Most people take his words literally and hold on to their investments for a long long time, often ignoring the deteriorating fundamentals. If you go back 10-20-30 years, you will realize that some of the companies used to rule the markets are no longer there in the top 500 companies list anymore. From the companies that were there in Sensex in 1991, only 9 companies are there till today. That means 2/3 of the companies have been pushed out. Think of companies like Bombay Dyeing, Hindustan Motors, Premier Automobiles, Mukand Iron & Steel and other such names which used to rule the roost back then.
The point I am trying to make is that if you follow Buffet blindly without understanding the context in which the great man makes a statement like holding forever, then you are headed for trouble. Buffet says you need to ensure that the management is a custodian of shareholder wealth, the business has a great moat. Also, Buffet buys when there is great pessimism in the markets regarding the company. He bought Amex and Coke during times when the companies had severe short-term troubles. That was when "normal" retail investors where selling in panic. He bought when he thought there was "value" but the franchises of the businesses were intact.
I think for small investors, a reasonable time frame for holding is one business cycle or approximately 3-4 years or till such time the business fundamentals are intact. Of course, that would mean keeping a constant vigil on the company and ensuring they are doing what they said they would. Buy-and-forget may be injurious to an investor's health.