Many sealers recommend waiting 30 days for the grout to fully cure. We offer a product called grout shield that can be applied right away and has an easy to use nozzle for application.
The sealer can and will wear off with time. To test to see when sealer should be reapplied, put a drop of water on the grout and if it beads up the sealer is still there. If the water dropped on the sealer does not bead up it is time to re-seal.