This answer is based in the practice of insight meditation.
You will know when to begin adding more time to your sitting practice. In the beginning quality is much more important than quantity when it comes to meditation. What does quality mean when it comes to meditation?
Quality means to make an effort to keep the mind in the present moment, so that one can observe the arising and ceasing of phenomena without the mind following after them or running away from them and getting lost.
By keeping the mind in the present moment one ties the monkey down. The mind is like a monkey jumping from branch to branch. By keeping the mind in the present moment is like taking the monkey in the hand and walking him to another area, an area that has only one tree. Now the monkey cannot jump anywhere.
In samsara, conditioned reality, the mind can never be satisfied with anything because all objects are impermanent meaning they cannot bring any lasting and permanent happiness to the mind. So the mind jumps from object to object but everytime it's left with suffering. The hardest thing in the world is actually to do nothing. So don't be too hard on yourself. It's the nature of mind to wander.
The tibetan buddhists have a saying that goes like this: "If the Mind wanders a Thousand times, bring it back a Thousand times".
Step by step we train the mind so that it will not stray from the present moment. It's usually a long process so don't get stressed about it. You might also not see any apparent changes at first but everytime you meditate you are changing something. You are untying the knots, slowly. Don't be fooled by not seeing any changes from day to day or even from week to week. There are changes happening but in the beginning they are subtle.
Results come not from how long one sits but from consistency, diligency and the ability to keep the mind in the present moment. It's like learning to play football. If one only plays once a week one will not improve that fast. If one practices let's say 1 hour every day then the benefits will be more solid and apparent. So to become good at any skill in life, one has to practice consistently.
Don't worry if you sit 30 minutes or 10 minutes everyday, just try to make an effort to do a little bit of meditation every day. Then naturally you will increase your physical and mental stamina and then you can begin to increase your sitting time.