First you have to pick the level at which you consider the capacitor to be "charged." This will be measured either in the voltage across the cap or in terms of RC time constants. There is no specific time at which the charge on a capacitor is said to be "complete," unless your instructor told you how many time constants to use. (Typically one, two, or three.)

I'll continue here so I can post images. Thanks for the kind words. BTW, I'm just an experimenter. Any electronics engineer knows more than I do. I have some hands-on experience from a lot of experimenting and blowing stuff up. When I was a kid, every time the house lights flickered and went dark, my dad roared like a King Kong and came after ME. How did he know??? :) But a week later he would bring home a few more used electrical/ electronic gadgets for my bro and me to play with. :) Back on topic, where are you in your study of electronics? For example, are you comfortable with Ohm's Law? Are you familiar with the RC time constant? Do you recognize the RC charge and discharge curves? (Images below) EDIT: The captions on the 2nd and 3rd images got stripped. Image #2 shows how the voltage rises across the capacitor in a series RC circuit. This is the charge curve. Image #3 shows the discharge curve, when the charge is bled off. More details later. Example circuit: If you place a 100k resistor (R) in series with a 40μF capacitor (C), the time constant is RC = ohms * farads. Normally we use engineering notation, where E3 means 10^3 (1000), and E-6 means 10^-6 (0.000 001) Paste the following into the Google search bar: 100E3 ohms * 40E-6 F Note that 100E3 means 100 * 10^6, and 40E-6 F means 40 * 10^-6 Google reports: (100E3 ohms) * 40E-6 farads = 4 seconds I.e., one time constant for this circuit is 4 seconds. Are you following so far?

Take a look at the discussion at this site: http://www.bcae1.com/capacitr.htm#demo This explains the electronics involved, and even shows a basic way to use a test lamp to show when the capacitor is fully charged.

There's a couple of ways. Either use a an NE555 timer chip with the RC component in the timing circuit or use a LM311 voltage comparator chip and compare the volts on the capacitor with a reference voltage.