1) the manufacturer's suggestion (i.e., are they daily, biweekly, or monthly contact lenses)
2) the health of the surface of your eye. If you have dry eyes, allergies, or have developed surface issues such as bumps under your upper eyelid (Giant papillary conjunctivitis), which is checked by having your doctor look under your upper lids, then you should replace your contact lens more frequently.
A contact lens is a prosthesis fitted to your eye, and you need to have regular eye health check-ups to be sure you are not damaging your eye by over-wearing them.
John S. Jarstad, M.D., F.A.A.O.
Jeffrey D. Gold, MD