Excerpted from Bogle on Mutual Funds by John C. Bogle, page 78

In most cases, a fund should prove its merit over a period of at least five to ten years. There are notable exceptions. A new fund introduced by an established investment management firm and modeled on its traditional investment philosophy should be considered. Another exception may be a new fund with specific investment objectives and characteristics that is part of a large complex. For example, if a fund complex offers a new balanced fund - and has provided good returns on its stock and bond funds over time - there would seem to be little hazard. Also, the inauguration of an index fund (and we shall see many of these in the coming years) poses few potential problems. But to invest in any fund without considering its heritage seems foolish.