How often should you reseal travertine tile floors?

Depends on cleaning methods used after sealing. Strong cleaners may strip sealer or weaken them. You can test to see if the sealer is working by opening a bottle of water, fill the cap with some of the water and pour it on some of the areas you are concerned about. See if the water stays on top of the surface for 10 minutes. If so, it is good for a while. If it start seeping into the stone floor, then it time to clean and protect again. Sealer vary as far as how long they last also. Their are some that claims to seal for up 20 years.