It is important to create some form of routine as it will aid you in sticking to yoga and being vigilant about practicing every day. Set an amount of time aside that you are willing to dedicate to yoga each day, 15 minutes is a good starting point, and decide when you are going to practice.
I suggest avoiding the I will practice when I have time today plan as usually you will miss it as something else pops up. I find the 3 best times are as follows: morning, noon and night. The benefits of each are as follows. Morning: It is an awesome way to start the day, it can keep you calm and energized throughout the day. It also gives you the time to meditate on the day ahead and what you are going to be doing. Noon: This can be an excellent time if you are looking to offload the stress of the day and unwind.
Often times we get stressed from all the madness of a busy morning, a quick yoga session can alleviate all of this. Night: An excellent way to unwind after a busy day. Offload the stress and pressures of the day and use it to escape work mode and enter relaxation mode. This will help you sleep better and improve your work/pleasure balance. Personally I like to practice once in the morning, before breakfast and once in the evening before bed.