When to replace a toilet's wax ring?
You only have to replace the wax ring if the toilet leaks. It's wise to replace it whenever you remove the toilet, though. It's not a matter of age, but the fact that a wax ring is intended to be a single-use item. They squish into place when you set a toilet, and that can't happen very well more than once. 

It's certainly possible that you achieved a satisfactory seal when you reinstalled the toilet. If you're willing to wait and see, and take the risk of (potentially smelly) water damage to your floor, go with it.