How do I limit the current drawn by a DC fan to increase battery life?
The fan will draw less current when it is given less volts. The current draw is roughly proportional to applied volts as is the speed. Consult the fan manufacturers specs and/or check on a variable lab supply to obtain a minimum voltage for reliable operation.

Lower fan voltages will mean that less air is shifted. Now make some PWM to control the fan and you will save battery power. If you run the fan at say 70% speed meaning about 70% voltage and about 70% current then power is halved if your proposed PWM system is perfect.

What you can do is connect your PWM control to a temp sensor so the fan speeds up when hot shifting its full specified amount of air and slows down when cold saving the battery and making less noise and increasing bearing life. Sure this is more complicated than a resistor but these days it is worth it when SMD is used.