Yoga is a long practice, and you must keep at it to reap the benefits of your efforts. It is worth you putting in the time however, and once you get into a routine, it becomes as natural as going for a jog. Yoga isn't seen as a quick fix, however, nor is it seen as an exercise which takes as little as 15 minutes.
The Hippodrome has recognised this issue, and created Lunchtime Yoga, which is a 45 minute class of either conditioning and relaxing Pilates (every Thursday), or a strengthening Yoga (every Tuesday) class. Each class starts at 12.30pm and ends at 1.15pm, so you have time to get back to the office.