I don't know about sewing machines, but my dh told me turning the regular light bulbs on and off is harder on them than leaving them on. I would definitely not be turning my machine off and on several times over a 2 to 3 hour period.
Why so concerned about the sewing machine light? I would just purchase a spare or two as "security" and use the sewing machine normally. I have a Bernina 1230 that must be close to 15 years old and have left it on overnight once in awhile. I suppose one would call my usage of it light to moderate, since there were several years when I didn't do much sewing, but the light bulb is still going strong! Haven't had to change it yet. As for energy consumption, I should think that is at most a few pennies per hour. Years ago I figured out the energy requirements of a tv left on for 24 hours, and I think it was something on the order of 25 cents per day! (Edit: I think my memory on that was wrong. I think I amortized the cost of the tv over its life, and *that* came to 25 cents per day. Not that I'm an authority on that subject......)