Relevant to American hams only: US amateur radio licenses are valid for ten years (10 year grant term). License term or renewal rules are likely different in other countries.
So a newly-licensed ham doesn’t have to worry about renewal for a long time to come. Various organizations will nag you via email, postal mail, and on QRZ when your license expiration is on the horizon (“This license expires soon. Renew Now!”). So if you intend to keep your license you should have plenty of warning and opportunity to renew. You can renew within 90 days of expiration but no sooner.
If, for any reason, you let your license expire, you have a two year grace period during which you may file for reinstatement.
However, you may not operate (transmit) whatsoever once your license has expired.
Renewal or re-instatement (within the grace period) is simple and straight-forward. No cost and no re-testing required. Unlike the old days, you do not need to prove activity for renewal (showing log entries). Make renewal application on the FCC ULS website. If you have trouble navigating the process, there are renewal services eager to do the work for you (for a reasonable fee).
Direct renewal via ULS should be very fast. Your status with new expiration date should show up on the ULS database shortly after processing. If previously expired, do not transmit until you see a new expiration date a decade away (much like your original license experience).
After the 2 year grace period has passed, the FCC will cancel your license and make it available for reissue. If your license gets canceled, your call sign is lost and you must pass an exam again to get re-licensed. After you have obtained a new license and call sign, you may apply for your old call sign as a vanity call, if it is still available.
Not all new hams are brand new to the game. Sometimes hams let their license expire due to neglect or lack of interest. More commonly a ham will find that a career and family interrupt their interest in amateur radio and they just let it slide. Then years— perhaps decades—later they catch the bug again and want to get back into ham radio. In some ways they are a new ham because many things have changed (rules®s, technology). On the other hand, the basics are familiar so they aren’t totally green.
There is good news for previous holders of General, Advanced or Extra Class licenses. If you can show that you held one of those licenses (FCC record or copy of license) you can get it reinstated by simply passing the Technician exam (Element 2).
The one disqualification is if the license was revoked for some violation, a rare situation.
Bottom line here is that if you held a higher class license many years ago and can prove it, you can get re-licensed for that class by simply passing a modern Technician exam.