While it's great to take a full yoga class, I like to incorporate yoga into my life in bits and pieces. Rather than waiting for a separate "yoga time" to stretch, I will often take yoga breaks during the day. This might consist of doing a gentle spinal twist in my desk chair or standing up and flopping forward in a loose standing forward bend. By incorporating small bits of yoga into your day, you can get some benefits without having to stress over setting aside a block of time to do it.
I do this all the time too, especially at my desk. It's great to get the kinks out with a few good stretches. I'm not sure these mini-sessions get you all the benefits of 30 or 60 minutes on the mat, but they certainly help you feel better.
Remember, the ultimate goal is the relaxed state you get at the end of all your "work". They always say that final relaxation is the most important pose.