While vegetarianism is a consideration in testing for B12 deficiency (as you are at higher risk). B12 supplementation is usually advised based on monitoring levels in the blood rather than your demographics.
B12 is a stored vitamin so it may take a long time (months to years) to develop a deficiency. Therefore you are advised to take more than your body needs to restore these stores of B12 and monitor whether your B12 levels pick up with regular blood tests.
Another consideration is that B12 is poorly absorbed if taken orally. While the standard daily dose is 50μg (micrograms), only a fraction of that is absorbed. The daily intake varies (higher for pregnant or breastfeeding females) but is between 2-3μg for all adults and these supplements should be sufficient to absorb enough B12 if you have a marginal deficiency. While B12 overdose is rare, doctors advise against taking more than the standard 50μg dose as more than that is not necessary (mainly due to cost concerns rather than overdose). 50μg is the dose offered by several supplement companies (here in NZ) and higher doses for "energy" or "natural alternatives" are not recommended by doctors (as there is no evidence that they are beneficial).
Another option is injections of B12. Some people have issues absorbing this vitamin so it is recommended if B12 levels aren't restored by oral supplements or if patients have severe deficiency at diagnosis. Since it is a stored vitamin these injections may be given weeks or months apart (depends on your blood levels). This has more risks of overdose but is still widely regarded to be safe and is done routinely for patients with a variety of conditions (including Chronic Fatigue Syndrome). These injections are 1000μg (weekly) or 5000μg (3-4 weeks) at most. Note that while these are higher doses, the average daily intake is only marginally higher than oral supplements and these are intended to restore vitamin stores in deficient individuals rather than meet a daily need in those absorbing it regularly in the diet.