As an endurance athlete and coach, I've never liked the term "off-season." It simply doesn't apply. We may not be racing, but no athlete I know takes the colder months completely off. In fact, many of us don't even stop racing!
Certainly, a break is needed after a top-priority race (an "A race") since such efforts cost so much mental and physical energy. During the buildup to our A races, we intentionally wear ourselves as thin as we possibly can. We have to put aside such pleasures as sleeping late and dessert. We tap deep into our reserves, and it's my strong belief that you can only mine that well a couple of times a year before it's dry. Sure, you could approach training and racing more light-heartedly, and you would probably never need a break. Intentionally under-achieving in a race doesn't sound like much fun to me, though.
Our A race builds are difficult not only for us as athletes, but for our friends and family, too. Our significant others have to pick up the slack for us at work and at home, and they miss us when we're off on long rides and runs. It's good for us to take breaks so that they can see us again, and we can do some things together that they want to do. It's nice to spend some money on stuff not directly related to endurance sports. Our friends and family will enjoy having a period when they can talk about something besides training, and they will like having something for dinner that's not spaghetti.
How long should a break last after an A race? It's different in every case, and it's largely determined by how difficult a build was on the athlete and their family. But while a break will likely involve some days off, it is most certainly not an off season. After a couple of weeks, most endurance athletes will start getting a little stir crazy and irritable. Near the end of my most recent break, I concluded that the guy who wasn't working out and was eating whatever he wanted simply wasn't me. While I'm not yet building for my next big race, I'm back to running, cycling, or swimming several times a week.
This gap between builds is the closest we have to an off season. Several athletes I coach are, right now, doing less than they did in the months leading up to their last big race and less than they will be doing in the months leading up to their next big race. They are letting their bodies, their minds, and their bank accounts recover from the beating that they underwent during the build to their last A race. The extra time athletes have now can be spent with family and friends; this less time-intensive stretch corresponds nicely with the holidays. I encourage the athletes I coach also to take this time to work on things that can't be done in the heat of a major build: trying out new gear, tinkering with their bike fit, doing some new sort of cross-training, changing up their diet, and mapping some new training routes. While it's a good time to catch up on backlogged work, it may also be a good time to read books that have been on your shelf or even [gasp!] cultivate a new hobby or two. It's okay to prioritize other things over training when you're in the gap between builds. The goal is to arrive at day one of your next training block fresh, healthy, and ready to roll.
A "off season," though? No. Of course not. If that's what you want, you're in the wrong sport. No goal-oriented endurance athlete—whether that goal is to qualify for Boston, qualify for Kona, get a seeded spot in the Peachtree Road Race, or PR at the local 5K—takes a whole season off given the devastating impact that such a long block of time off would have on one's fitness. Taking a little bit of time to re-balance your life, try new things, and honor those who helped you accomplish your last goal, though, I highly recommend.