Flexibility/mobility work should always be about the quality of movement, not how many stretches you can get in a certain time. Even if you wanted to do a full body stretching session, it's important to focus on the best bang for the buck movements rather than always trying to do a bunch for everything. Unless you have significant flexibility/mobility issues you're trying to correct, a handful of basic movements should keep you covered and loosen up key areas.
One other way to "sneak in" some flexibility work is to basically superset it into your regular lifting sessions, alternating an exercise with a stretch for a different bodypart. Bench press and quad stretch or squats and chest stretch, for example. Sometimes called "fillers", it's a great way to maximize rest periods when lifting.