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..???

Yes drawing energy from the earth has worked well for many I know, but I use personal energy all the time, at the begening I would drain a lot but as I got older and stronger I don't drain unless I go all out for something huge, which rarely happens. But when I do need those huge thing I draw energy from "The All" and or "The Elements". If you have any clear quartz or rutilated quartz, they hold energy rather well, but they leak if you don't keep them contained in either silk or metal. The reason why I brought these stones up, you can charge and fill these with energy to hold until you need them. And rose quartz is good for draining I'm told by a friend of mine. It's good to drain your energy every once in a while to clean your chakars and what not.