I believe there are actually two phases to the “off season.” Phase one is the rest and rejuvenation period that lasts from 4-6 weeks following an athlete’s last event or race (this is where I am at the moment). Phase two begins around 8-10 weeks after the last event and is the time to start your improvement plan for the following season. This might mean working on skills (e.g., stroke mechanic improvements in swimming or bike handling skills), or perhaps upping your cardiovascular endurance with longer, slower bike rides and runs (1-3 hours in length). And, it definitely means weekly strength, mobility and flexibility training both to improve muscle control and power.
Strength training with specific functional movement exercises can also to make sure you are less likely to injure when you do resume higher volume/intensity training. I like to call this resiliency training. Phase two of the off season can last anywhere from 3 to 6 months. In reality it takes only about 4-6 months to specifically prepare for an endurance event so there’s no need to rush things. I find that the more time I spend strength and agility training in the off season, the more likely I am to feel ready to resume longer swim/bike/run training sessions when the time comes.