How often do you calibrate your refractormeter??

[jhj0112]

I have used have my refractormeter for nearly a year (10 months) and never thought of calibrating it because it seems to work well..

How do you know it's time to calibrate or how often do you calibrate it??

on google, some says they calibrate every time you use, others say they have not calibrated it for over 2 years..