I update Calibre maybe every 10-15 versions or so, or when there's something new added that I find useful. I love that Calibre is constantly updated and improved, showing real dedication of the team behind it and that it will probably be available just about forever for ereaders.
But that doesn't mean you need to update every week. I certainly wouldn't go 2 years though between updates, there are a lot of changes that happen to the firmware on ereaders that could end up not playing nice with a 2 year old version of Calibre.
I also don't blindly update the firmware on my Kobo Glo either, I'm staying at 2.5.1 for now for various reasons. A lot of people don't immediately update unless there's some improvements.