User experiences - life span of optical level sensors?

I will preface this thread by acknowledging that it is fully clear and disclosed by GHL that these items have a finite life, so I do not expect them to last forever and this thread is not designed to be a complaint.

What I would like to understand is other users experiences with using the optical level sensor and how frequently you replace yours?

From my experience, using the for ATO purposes, I roughly they last 1.5 - 2 years before they fail. My maintenance regime is to clean the sensor (with a cloth) every couple of months.

How long do you guys and girls find yours last?