First: What is the part number of your optic?
Second: Did the optic come with a battery? If it did, and that's the battery you used, it was probably a little old to begin with. I had a similar issue.
Third: What setting did you have the optic on? If you have it on the top two settings, cut the expected battery life by 75%.
Fourth: Is this optic used for self/home defense? If it is, replace the battery every year regardless.
I have a Primary Arms MD-ADS sitting on my desk. I put a new battery in it on 12 Mar 2016. I know because I put a label on it with that date. I set the dot to the middle setting and have not adjusted it. As of today, the dot is still working. That's 2 years 8 months on a single battery. That's plenty good enough. If I were using this for self or home defense, I'd put a new battery in it and be confident it would be on when I needed it.