Minimum amount to keep fitness in the off season

For the swim and bike, you can likely mitigate the losses (unlikely you can maintain) with one quality session per week.

For the run, not only is it unlikely, it is a very bad idea to run once a week for an hour at a higher intensity. You would be much better served running 3 times for 20-30 minutes easy than trying to hammer one hard session.

Shane

Why not pick your weakest sport and focus all of your attention on it.

That's what I'd like to do (it's swimming for me) but I'm afraid I'll lose all the gains in running and biking.

Just something to keep in the back of your mind.

It's easier to hurt yourself jacking up your time/distance running than it is on the bike. Next year, you'll start ramping up for a race at some point, if you haven't been running, you'll have to start gingerly.

It's totally fine to let your fitness go. I usually take off at least a month at the end of the season. It gives your body some needed recovery time and its nice to have all the extra time to do other things. Fitness comes back quick, although, I must say that I've never gone longer than a 6 weeks of zero training.

All the articles I've read say that 2 weeks with no training will start to kill fitness. I would keep it with one workout a week of each sport just to keep it going and so you aren't practically starting at square one again.