First of all, all subs are “nuclear” nowadays, in the sense that they all use nuclear reactors to generate their power.
You guys are specifically talking about “boomers”, subs which carry SLBMs, missiles launched from submarines that often (almost always) contain nuclear warheads. Their entire mission is just to sail around in circles, more or less, out in the ocean and be ready to launch their payloads when they get the call and the codes.
When I did a tour of one they didn’t mention anything about how often they are taken out. Tritium, however, has a pretty short halflife, and warheads are only good for so long before there is not enough tritium left for a decent nuclear reaction. If I remember correctly, the Boomers are often out for 2-3 months at a time. They carry dozens of warheads, and I doubt they would want to replace them all every single time they come back, so my guess is that there are probably a few warheads that are taken out and replaced with certified “fresh” warheads when they come into port.