This has become more difficult in recent years:
In order to renew your license you need to have either sailed X number of days in the past X years, or you need to retake a test. For deckies you will also need to retake your Radar endorsement.
Now, along with the license you also need to renew your STCW certificate which has different requirements than the license. The latest requirements mean that if you are not sailing you need to go retake the Basic Safety Training course which is usually around 5 days long. The cost is variable depending on where you take it.
I personally had to go through this when renewing recently because I was in med school. I ended up going to MITAGS in Baltimore one night to redo my radar cert. I went to the CG station there to renew my license. A few months later I went back to MITAGS to take the 5 day BST course (I think it was around 1,000 bucks) so I could then renew my STCW cert. With all of these I would still be able to go and sail as a Third Mate.
I'm sure this will get more and more difficult to accomplish as the years go on. On active duty you would need to take leave to do the course, but it can be done. Once you have renewed once to meet the KP graduation requirements it would be possible to 'renew for continuity purposes' which would make the license part easier, but then you would likely lose the STCW which may be more of a pain to get than the license.
If you fail to renew your license you would have all sorts of trouble getting it back.