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

Excluding exceptional situations, how often a vegan should make those tests?
I mean, what is the minimum times per year (for example) to ensure a vegan has a good nutrition plan?

As a comparison, I 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.