Making changes to the body's tissues or altering movement patterns takes time. How you distribute that time matters. Improvements happen not only to the body's tissues, but also to the way that our brain signals those tissues and coordinates our movements.
The total time that you spend when you stop to work on flexibility or mobility does matter. You must perform a sufficient number of stretches or movements and do each one long enough to see a change. However, once those minimums have been reached frequency takes front and center.
Try looking at it this way. You could brush your teeth once per week for 14 minutes straight. On the other hand, you could brush for one minute twice per day and still only brush for 14 minutes each week. A different frequency and duration of the same activity for the same total amount of time can have a VERY different outcome.
Flexibility and mobility work much the same way. Regularly sending brief signals to your body that alter how you move has a cumulative effect on how you both feel and move. Between sessions, our body tends to revert back to its old patterns and positions. By performing your routine more regularly you "grease the grove" making it easier and easier to regain prior progress while also further alter your tissues and movement.
As a personal trainer, both here in Sienna, Texas and online, I make sure that my clients get great workouts when we meet. However, the small changes we make to my personal training clients daily routines have just as significant an impact. Each client has different challenges so it is important to find the movements and stretches that will have the greatest positive impact for each one of them.