My PRO is a year and a half old. I keep it burning all the time. Took the rifle out to the range and oops, it was not working. What a let down. Ordered new battery and wow what a difference it made. Maybe the battery that came with it was really old and drained.
It is a good habit to change batteries every year, even if they are supposed to last for several. Change 'em out the same time you change the batteries in your smoke detectors at home. They cost almost nothing - it is just cheap insurance. Very simple to do and ensures you are always getting peak performance.