First, let me say that you should always follow your floor manufacturer's instructions regarding recoating and refinishing your floor. But here are two general guidelines.

1) Depending on the traffic of your floor, you will probably need to recoat a urethane finish every 2-5 years. Be sure that any waxes or oils have been thoroughly stripped from your floor prior to recoating. Otherwise your coat will not adhere.

2) Your floor will probably need to be sanded down and refished every 10-15 years, again depending on traffic. This major process can be done many times to a real wood floor, but perhaps only a couple of times on a veneer. If you're not sure what you have, consult a floor specialist.