Just some anecdotal evidence ... Last year I had my B12 tested, after about 8 years of veganism without regular and consistent supplementation of B12. I took supplements now and then, but not regularly.
The value was at the lower end of the acceptable spectrum with about 200 ng/ml.
By taking supplements (Jarrows 2,500 mcg daily) over a few months I could increase that level to about 850 ng/ml, so my doctor suggested I can now take them maybe once every two days or once a week.
I am always dismayed when some vegans I know on facebook feel they have to point out (when people ask about supplementing B12) that they have been vegan for decades, never supplemented B12 and are still fine. Indeed, that might work for some people, that they get enough from fortified foods without ever having to think about supplementing, but it is IMO very dangerous advice to vegans in general.