This is so far into false economy it's hilarious. Consider a few things:
- a bulb lasts a couple years of heavy use and costs $5 to $20, compared to the $2000+ you'll probably spend on film, paper and chemicals to do that much printing
- running at lower voltage lowers the temperature, which makes the light redder, which means lower contrast from VC paper. Once you've corrected the contrast with magenta filtration, your exposure times will be longer - probably longer than is necessary to make up for the "increased" bulb life
- bulb life in an enlarger is related mostly to number of power cycles (due to inrush current at startup), not total burn duration. So you're optimising something that isn't even the dominant factor for bulb life.
If you want longer bulb life (why? it's an irrelevant cost compared to film, paper and chemistry) then put a mechanical shutter on your enlarger and switch that instead of the bulb. Or do LED exposure.
Or seriously, quit worrying about irrelevancies. This is like worrying about brake-light life on your car.