I experience approximately one crash per day (one day = 2 hours of work with SONAR), I think it's too much. I've not investigated to know why, but my projects are quite big (40 tracks, 15 synths, more than 100 FX). It often happens when I am acting very fast, for example while testing lots of patches or lots of FX settings. Sometimes it happens when starting or stopping a project, or when it reaches a certain point.
As someone says elsewhere, I am now used to typing CTRL+S every time I do a significant change in the project. But I also do this with Word or Excel, because sometimes you've got crashes too.