When you increase your stamina, you can increase the amount of time a certain muscle or your whole body can be exercised.
In yoga, we have the choice of holding our poses for a longer period of time, and the longer we hold our pose (with correct posture, of course), the more muscle endurance we are creating.
Instead of staring at a stopwatch, which may interrupt your flow, you can measure the length of time you are keeping a pose through the number of breaths you take. You can set a goal, for example, holding Chair Pose (Utkatasana) for five breath cycles (one cycle being an inhalation and an exhalation), and the next time you practice, you can aim to hold the pose for ten breath cycles, and so on.
Keeping a record of how long we can hold our poses for in a journal is a helpful tool to track progress.
A few fantastic poses that benefit from being held for longer periods include Plank Pose (Phalakasana), Warrior II (Virabhadrasana), and a Utthita Parsvakonasana (Side Angle Pose) variation where both arms are engaged and raised in order to workout the obliques.