Personal anecdote, so sample size of 1 and YMMV, but 5 months ago I started doing exercise in the mornings that has done wonders for my concentration and has helped me drop 25 pounds without changing much else in my life. It takes 15-20 minutes, 3-4 times per week and I literally don't have to get out of bed to do it.

After being frustrated by my air consumption on a scuba dive, I got interested in free diving (apnea) training. I found an iOS app called Apnea Trainer that looked promising and started doing the breathing exercises. I've gone from being able to hold my breath for just under 90 seconds to breaking 6 minutes for the first time last week. It also revved up my metabolism (like I said, I'm down 25 pounds) and I've gone from having borderline hypertension (with a family history of stroke) to having blood pressure that's almost worryingly low (usually around 100/50). And, like I mentioned above, it's been good for concentration and stress levels as well.

Wet training (in a pool or body of water) requires instruction and a partner, but dry training is pretty safe to do on your own as long as you don't push yourself beyond your limits on a regular basis.

It's amazing how little exercise it takes to do the trick. Personally I feel like it's important to do something that opens the lungs and gets the heart beating, so cardio of some sort is what I choose (sometimes skipping rope). But some sort of all-body stretching (yoga sun salutation or similar) is actually enough.

I swim a mile. I've been doing it for about 15 years so it's automatic and almost meditative, plus a full-body workout. Surprising how much head work I can do in that half hour.

I also paint, but you can probably insert any totally immersive creative pursuit here (music is probably more common amongst hackers). It completely switches brain hemispheres and unlike programming, my mind does not wander at all when painting, and I can literally go to another place for hours. Unlike when swimming, I can't think about programming or anything at the same time, I am completely absorbed in the painting. Paul Graham has lots to say on the subject.

Exact numbers don't matter. It depends on your physical status. I find that 20-30' of exercise totally changes my mood and stress levels without exhausting me, allowing me to do actual work afterwards.

Cycling in the morning does it for me. I feel like a totally different person when I skip the cycle and get public transport. Even if it's as short as ~2 miles to where I'm going, I find getting the body going before the mind helps me focus.