First there are some basic flight requirements. You don't need an instructor to complete these. It basically boils down to 3 take offs and landings every 90 days to be current to cary passengers. Here is the full legislation.
The license its self (as the laws currently stand) is good for life, barring you having it revoked for some reason you will not need to fly your check ride again (unless you go for something like an instrument rating or a higher license).
Until recently here in the US you also needed a valid medical certificate. However recent regulation change has altered it so that a VFR PPL for most aircraft can be had as long as you have a valid drivers license and have not had a prior medical revoked.