How much should one exercise for a normal healthy life?This answer depends on your genetic disposition. Some folks can work out like mad, with little result. Others can do the minimum and get great results.
Why not go for the minimum? Clarence Bass, who in his 70's has a kick ass physique, works out two days a week. See his website, cbassdotcom
You could also go for a weekly all body workout - say squats, pull-ups, burpees, pushups, jumping jacks, running in place, plyometric squat jumps, and thrusters (squats that go straight into overhead press, done with a pair of dumbbells) in exactly 4 minutes, following a Tabata protocol. You will need a clock with a second hand, a pair of dumbbells and a pull-up bar to do this.
Warm up first, jumping jacks, walking in place, twists, bodyweight squats.
Then start your Tabata.
Go 20 second all out, followed by 10 seconds rest. Do each of the above parts one time for twenty seconds.
Repeat 8 times. Total of 4 minutes.
It. Will. Probably. Kick. Your. Ass.
If done correctly of course.
You may need to back off, try easier parts, or harder parts to match your level of fitness.
For purity, try this on a track. It will make most Olympic athletes, Navy Seals, NFL players, and Crossfit athletes huff and puff and barf like an 80 year old, if done correctly. Which is 100 effort for 20 seconds, followed by 10 seconds rest.
You can't get much more minimal than this workout, and yet it is hard to find many workouts that are more effective. or brutally efficient. Three times a week is more than plenty, but once a week will do.
Here is an example: