I read (e.g. in this vitamin B12 question) that it is important for a vegan to monitor some nutrient levels by taking blood tests periodically.

Excluding exceptional situations, how often a vegan should have those tests?

For example, what is the minimum number of times per year a vegan should seek a blood test to ensure they are obtaining correct nutrition?

I have heard that it is advised to see a doctor at least once a year to make sure everything is okay, even if I feel good. Is this accurate?