It's confusing, I know, partially because real studies often are mixed with assumptions or 'facts' that aren't documented. Due to all the 'B12 destroyers' in our modern world I guess most long-term vegans either check their levels every year or so or take some supplements 'to be on the safe side'... although almost no vegans or non-vegans will ever be 'on the safe side' re. B12 - both because we know too little about what's safe and because even if people consume enough B12, they may also be exposed to a lot of B12-killers or have an absorption problem (the latter is rare, but there's probably no way to find out if you're among the people who actually have a B12 absorption problem without actually testing yourself).
B12 is a a hot topic, and there are fanatics on both sides of the discussion - some people insist that as long as you eat enough green, organic vegetables or seaweed you're always safe (wrong!), while others claim that unless you take B12 daily or weekly, you'll necessarily living a dangerous life (also wrong!). So - you're confused for good reasons, and it doesn't get any simpler when looking at all these studies and sites that doesn't come to the same conclusions!