I got a desk treadmill to improve my productivity 

It's perfect! So sturdy.

My desk is 24" deep. But the treadmill only goes under about half of that. I'd say about 6" from the front of the desk to where the actual tread part of the treadmill starts (there's the screen and other stuff behind that, you have to account for of course, but that varies by model). I found that it doesn't have to go very far under the desk because when walking on a treadmill, your front foot doesn't extend very far with each new step because you are simultaneously being pulled backward at the same time-if that makes sense.