I exercise 3 times per day and am working my way upto 10 mins each time. I use a rower. It's comparable to walking up a steep hill.
The idea behind it is not to improve cardio vascular functions or to lose weight but to improve digestion. There is a short term release of various hormones and in return, enzymes which work on a number of things such as fats and also improve insulin sensitivity. The effect is short lived so a couple of longer sessions per week wouldn't do it for me. The short sessions do have a cumulative beneficial effect though and, from the studies I have read, I should be targetting burning 1200 kcal per week through resistance exercises. I'm at around 40 per session at the moment so, 120 per day or 840 per week. The effect is noticable though. It drops my BG faster than if I don't exercise and, if I have a post meal nap, my BG goes up considerably. At about 30 mins per day, I should start to get some cardio vascular benefit which in turn should increase my metabolism which ought then to help a little with weight loss. Generally though, even with moderately improved fitness, people lead more active lives, do more and therefore exercise indirectly. I have notice that I am spending more time in the garden, shifting and burning rubbish, tidying up, cutting the lawns and hedge etc. I'll be happy if I can get upto 3 x 15 mins per day, each time about 1 hour after every meal.