The coin cell lasts for its shelf life. The MOSFET gate draws picoamps (too small to measure) once the gate capacitance has charged up. If the trimmer pot is a 470k, then the coin cell supplies a current of 1.5V/470k, or 3.19 microamps. If you use a 1 meg pot, it will draw 1.5 microamps. In either case, that's so low that it will take years to drain the coin cell. Say, for instance, the coin cell is rated at 150 mA-hrs. The 470k will drain the coin cell in 47 thousand hours, or more than 5 years. The 1 meg pot will drain it in over ten years.