As someone with an MBA in finance and a licensed financial consultant, the general rule is, if your window is greater than 5 years, hold on to it. There has never been a 10 year period in the history of the financial markets where the markets were lower than when they started. Do a search for the Andex Chart and you'll see.

you can also switch the type of funds you hold though. Of course, there are thousands out there, each of which designed to do different things to accommodate different needs, risk tolerances, time frames, etc. As CGS said, an index fund rides the market, whereas other types try to beat/time it, which is very hard to do.