well since im new here and I feel like a peon compared to some of the genius minds here, I'll explain some more about resistance, capacitance, and time because its something I actually understand!
[tau] = RC you knew that.. but 5 * the time constant of the circuit is considered the "steady state" in which a capacitor doesn't charge anymore.
As time increases, the capacitance increases exponentially according to a certain equation: Vc(t) = Vf( 1 - (e^-t/[tau]) ) where Vf is the total voltage, t is the time in which you would like to observe the voltage "on" the capacitor, and [tau] is the time constant.
since an RC circuit is (when reduced according to Thevenin for all of you critics, hehe ) a series circuit, the voltage across the resistor will be the total votage minus the voltage across the capacitor at that time, so as the voltage across the capacitor rises, the voltage across the resistor falls.
Probably more than you wanted to know, but hey I figured I might as well make a good impression, because I'll probably be sounding like a jackass rather soon.
time constant is appliable to condensators and other fields of el. engineering, but it doesnt reveal whole RCT function.