This is the amount of exercise you need to maintain current fitness levels

Physiotherapist Håkon Hov, encourages you not to fall into the trap of training at a low intensity because then you need to train more - remember that intensity trumps quantity.

Health outcomes are perishable goods, train smart

It's a well-worn expression that exercise is fresh produce. And to a great extent, it is! During the most hectic periods, you can therefore “put the freshly produced health equity in the freezer” and in this way safeguard its shelf life. It takes a lot less to maintain a good shape than to create a good shape! It depends on your starting point, but the vast majority of us manage to maintain both strength and endurance in a hectic period of two to three weeks, with one interval session and one strength session a week - as long as this is performed heavily enough. 

You should train smartly and efficiently 

Håkon Hov, fysioterapeur
Don't fall into the trap of training at low intensity because you will have to train more frequently - remember that intensity trumps quantity.
This is the whole point of intensive long intervals and heavy strength training; you get much more out of the work you do.

How does your starting point affect your progression?

How quickly you can improve your fitness depends on your starting point. In many ways, it is like being 'the first person to be out of breath.' The worse shape you are in, the less work it takes to get out of breath, and you will make faster progress.
The same principle applies to strength training. The weaker you are at the start, the faster you will progress.

Take more control of your time

Fortunately, the whole year is not as hectic, making it easier to 'take control' of your time. Use this time wisely for your training. Build yourself up with strength and conditioning exercises. You will be even more efficient, even during busy periods, if you know what you are doing. Here is an effective training program you can follow.

With 2-3 training sessions a week focused on the 4x4 principle for intervals and strength training, you are likely to increase your endurance (vo2max) and achieve a biological age of 20. 
Follow the science and do what it says is the smartest and most efficient workout. Train a little harder because then you can tolerate a hectic lifestyle and still maintain your form.