spell casting???, when you know when your spell has power Options

my question is simple yet Im just wondering how you guys all know how much power/energy to summon for a spell..
sometimes I feel that spells drain me quite fast..and I was wondering do you guys know when to stop sending energy into the spell and into the world..I mean do you have an internal alarm that go's off or something , how do you know when the energy has been fully sent out with the spell..???

i usually draw in energy from the moon during the nighttime and the sun during the day

if i need grounding of course i use the earth

and grounding gives you an extra boost of energy as well

i like to give some of that energy back to the earth though...when i'm done with casting