Magnets don't possess a half life. You're not dealing with radioactivity.
You're seeing evidence of the magnetic field collapse of the inductor, the magnetic field rises and falls over time, inductors resist current change.
The best way to determine the charge on the capacitors is to monitor their voltage.Q=CV.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html
This is a kind of coil gun you made.
Safety:
Gary, why are you playing with these industrial level voltages and currents when you don't understand the theory?
You can cause serious harm to yourself if things go wrong! That coil is going to generate serious inductive kickback upon collapse of the magnetic field. It's probably arcing across the switch. If it can't discharge somewhere it's going to make killer flyback voltages! You better not have a pacemaker and collect a zap from that coil!