Ultimately how much any particular capacitor is charged depends on the total Coulombs it holds. The capacitance and the voltage level at which you consider it charged dictate the number of Coulombs you need to run thru it to "charge" it from the totally discharged state.
Charging a capacitor thru a resistor from a fixed voltage source is only one of many possible ways to dump the required Coulombs onto the capacitor. In that case, the voltage is a exponential approaching the voltage source value. In some cases the circuit may look more like a current source, in which case the capacitor voltage would rise linearly. In other cases the circuit could be non-linear, so all kinds of unusual voltage profiles are possible.