What does it take to maintain a private pilot certificate? What, if any, requirements are there to maintain a private pilot certificate? For example:


Do I have to renew my certificate after a period of time? E.g. annually, 5 years, 10 years
Do I have to receive a minimum number of continued training hours?
Do I have to fly a minimum number of hours?


For instance, say I had enough money/time to go out and start the process of getting a private pilot certificate tomorrow, but I didn't plan to make use of it in the foreseeable future, would it cost me more in time and money than if I waited until I had plans to use it?

This is assuming I live and fly in the USA.

Related Question:
How much, generally speaking, does it cost to learn how to fly?

Edit:
Note I understand the safety aspect of not practicing and "getting rusty", for lack of a better term on my part, and that practically a certain amount of retraining would be required (though it seems that it is not legally required). The retraining is not mandated and I would need to retrain until I felt proficient enough technically and practically. Retraining would include familiarizing myself with the controls, instruments, radio, physics, etc.

My main concern was that (a) at some point I my certificate would expire and I would have to go through the process again, making the initial effort practically useless, (b) that I may have to pay a significant amount of money to keep the certificate from expiring, or (c) that I would have to commit to a certain number of hours (instructed or solo) per year to keep the 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.