calculating the input current
When reading an analog voltage an analog pin has a very high resistance - usually very very much higher than the resistance of any potentiometer used, and input current to A0 can be regarded as zero for practical purposes.

For practical purposes the current drain from the supply will be that which flows from 5V to ground via R1 &amp; R2.
I = V/R = 5V/(R1  + R2)   

As for a potentiometer the sum of R1 + R2 is constant, the position of the pot-wiper makes no difference to current draw.

eg 10k pot. I_supply  = V/r = 5V/10K = 0.5 mA.
An I_A0 ~= 0



The upper value of a potentiometer which should be used for an analog input is covered by the uC (microcontroller) data sheet. The key parameter is uC input bias currents flowing in the potentiometer and causing small voltages which affect the analog reading. A rule of thumb is to keep such voltages under about one LSB (least significant bit) of the ADC.