worth noting that its probably less about time and more about energy- so make it easy to get 30m of exercise. do something you like, make it near where you are, etc...
The funny thing is, since I started my energy levels increased. I can sustain much more time focused on a subject than before I didn't do any exercise. So, the first two or three weeks will be painful (physically), but afterwards, trust me, you will feel with so much more energy and less tired after work.