What does it take to maintain a private pilot certificate?
A license, once granted, is good forever (barring some enforcement action).

But, you need more than just a license to fly.  To use your license, while flying solo, you also need a Medical Certificate (if required for the aircraft or flight rules) and a Current BFR (Biennial Flight Review).

Medical Certificates are good for 5 years until you're 40, then good for 2 years.
Its a quick trip to an FAA approved doctor to examine you for any obvious medical conditions.

A BFR is a review with an instructor every two years to make sure you're still proficient and safe.  If all goes well, it takes about 90 minutes.

However, if you want to fly passengers, you also need 3 landings within the previous 90 days.

If you want to fly passengers at night, you need 3 landings to a full stop, at night, within the previous 90 days.

If your 90-day landings have lapsed, you can quickly fix it with 3 trips around the pattern, taking off and landing each time (touch-n-go during the day, or full-stop at night).  Then taxi back to the ramp, pick up your passengers, and go on your way.