Most vegans get their vitamin B12 through supplements.
When you look at most vegan vitamin B12 supplements, they have serving sizes over 1,000 mcg (1 gram), even though the RDA of B12 for an adult is only 2.4 mcg.
According to a study that looked at how much B12 is absorbed by the body, the amount depends on the dose.
They found that 56% of a 1 mcg dose was absorbed, but the percentage went down as the dose increased.
Here’s a table of their findings.
For a 1,000 mcg serving, which is pretty typical in supplements, approximately 1.3%, or 13 mcg is absorbed.
This is still well over the RDA, which is why you don’t necessarily have to take these religiously every day (even if the bottle might recommend it).
Keep in mind this assumed that your body is healthy and you don’t suffer from malabsorptive disease.