Yes, of course you add all the currents drawn by all devices in your circuit.
The current drawn from the battery can be the same if there is no regulator between the battery and your circuit. If there is a linear regulator you ave to add its quiescent current to the circuit current to get the battery current.
If there is a switching regulator between the battery and the circuit, then this will draw a constant POWER from the battery. Its input current (=battery current) can be calculated if you know its efficiency and output voltage and current:
Ibat=(Vcircuit*Icircuit)/η/Vbat, where η is the efficiency of the regulator. This current is an approximation, since it increases as the battery discharges. Plus, the efficiency of the regulator can also change as the battery voltage or output current change. But it should give you a fair ESTIMATE of the input current.
Once you have established the current the battery needs to supply, just use the formula given by checkmate to calculate the time.
If the circuit current varies, which it probably will, you have to use a best estimate as to how much average current it draws.