Any length of time that you have is the short answer.  The important thing is to do some every day.  2-10 minutes of Qigong is better than zero minutes.  If you have 10-30 minutes, then go for it!  If you have 60-75 minutes, have at it!  If you can arrange your schedule to do 20-35 minutes every day – fantastic, you will love it and you will feel good doing it.  If you are having a particularly busy day, then take as some short qi breaks.  If you have been sitting a lot, then getting up every 45 minutes and doing 2-5 minutes of Qigong will be very helpful.  Those short qi breaks make a big difference!  Whether you stand on your feet a lot, have other work-related physical stresses or you are doing heavy lifting with your brain, short qi breaks are probably the essential minimum.