To add on above mentioned information by @"Rakhi-Certified VK Teacher" and @"Puneet-Certified VK Teacher" , please know that working for 30 minutes is a scientific mechanism of VK. VK is made so, constructed to work for 30 minutes on its own. You don’t have to sit and keep working as in other available healing modes, like Reiki or any other such practice.
Like Rakhi said, You use water from tap ? and that water comes from tank of water ? But you use water according to your need. You can use for washing only hands or washing clothes. Washing hands would take less time but washing clothes may take much water and more time. But the mechanism of a tap and connected tank is to provide you water.
Same way VK can provide you energy for any intention. It’s upto you if you want to wash hands or clothes or vegetables. Means it’s upto you that you want to use energy for health, wealth or any other issue.