Single-Engine Piston class rating is valid for 2 years and requires either an LPC to renew or 12 hours experience in SEPs in the second year. (You also need an instructional flight, but can count any LPC in its place -- won't be a problem if your ATPL is up to date)
Multi-Engine Piston class rating is valid for 1 year and must be renewed by LPC.
All turbines are now individual type ratings, renewed by LPC.
Different rules may apply if your ratings have expired by more than 5 years.