The duration of holding time depends upon the individual's capacity or strength in the current time and yoga pose. However, in general, it's recommended that you try to hold one post for 15-20 minutes in the beginning.
Once you start practicing yoga on a regular basis, you will feel your strength increase within a few days. You might increase the holding time with practice, whenever you feel like increasing.
It's crucial to take care of your body’s strength and hold it according to your strength.