It depends on the type of blood test, on one's health problems, and on one's diet and supplementation. Each test should be carried as frequent as it is needed.
Nonetheless, complete blood count (CBC) and Comprehensive metabolic panel (CMP) should be carried out yearly by everyone.
If preliminary blood tests come out bad, one may carry the blood test more frequently afterwards, to check whether diet/lifestyle changes have improved the parameters.
Other blood tests, specific for potential health problems in vegans, may be performed more rarely (or more frequently), depending on multiple factors, as listed below.
- previous results from the same test were on the desired interval, at the edge, or outside of the interval
- how well and often you supplement for a particular (potential) vegan diet deficiency e.g. B12, folate, vitamin D, DHA
- particulars of a diet e.g. high in plants containing oxalates (e.g. kidney or urine tests)
- after a drastic change in dietary preferences
- male or female (or female wanting to become pregnant): e.g. iron, folate
- known genetic or lifestyle liabilities (such as low exercise): e.g. blood lipids
Follow my answer to a related question for more details on each common blood test for vegans. It follows that the tests should be taken at least once, to have a good sense of one's health parameters, and decide afterwards (with a specialist preferably) when to retake them.
Disclaimer: not medical advice.