This would be from basic energy manipulation. Hold the rock in your hand. Concentrate (or even meditate) on the intent you want the rock to help bring about for you - being that there is a specific intent. Visualize how the rock will help you by bringing about the intent. When you have the thought and visualization clear in mind, start sending your energy through your body to your hands. From there, push out the energy from your hands and then into the rock. Continue this, along with thinking on your intent and the visualization, until you feel you need to stop, you get tired, or you feel the rock will not accept any more energy at the present moment.
As for how long the energy will last, that is debatable. Think of it this way: a battery will lose energy when in use, as well as when sitting around unused, but not as fast as if being used. No matter how much you close all of the holes you can, energy finds a way to leak out. Also keep in mind that it could also draw in negative energies whether by you, others touching the rock, or by some other means. So every now and then it would be good to cleanse/purge out the energies (particularly negative energies) and recharge the rock with your energy.