At least once annually.
If your business moves at a faster page, check your brand every time you re-evaluate your marketing plan – whether that's twice a year, quarterly or even monthly. Doing a quick check-in every time you're looking at your marketing enables you to make sure that your brand doesn't get off-course.
A brand is a bit like a ship. If you're steering it, and you veer off course a tiny bit, but catch it soon, it's easy to correct. But, if you let it go off-course for a while, it takes more and more effort to get it back on track.