If i lower voltage drawn from a battery will it last longer?
If I have it connected to a 12v battery and use a step down converter to convert the voltage to 6-8v, will it draw less amps from the battery and be able to run longer?


If you reduce the voltage to the pump the current drawn by the pump will reduce too. Since power, \$ P = voltage \times current \$ the power drawn will be reduced and the battery will last longer.

You need to estimate the number of watt-hours (Wh) your battery can produce and using that and the pump power (watts or W) you can calculate the run hours.

Steps:


Get the ampere-hours (Ah) rating of the battery.
Multiply by 12 (volts) to get the Wh rating of the battery.
Measure the current drawn (I) by the pump at a particular voltage setting (V).
Calculate the power used at that setting using P = VI. Answer will be watts.
Divide battery Wh by pump W will give you the run hours.
Multiply this by your voltage regulator efficiency.


Worked example:


Battery is 12 V, 10 Ah.
Pump draws 2.5 A at 6 V.
Voltage regulator is 85% efficient.


Battery capacity = \$ 12V \times 10Ah = 120~VAh = 120~Wh\$.

Pump power = \$ 6V \times 2.5 A = 15~VA = 15~W \$.

Run time = \$ \frac {Battery~capacity}{Pump~power} efficiency \$ 
\$= \frac {120Wh}{15W} 0.85 = 6.8~h\$.