I think it's the intent and commitment that defines it. Someone who does not, as a rule, consume meat or animal products is vegeterian. I think that's true if they do it for one day or a thousand, and IMO it's also true if they occasionally consume an animal product due to necessity or ignorance or mislabeling.
By contrast, ~80% of my food intake right now is Soylent. I could make that 100% if I wanted, and I suppose that would qualify me as vegeterian; Soylent contains no animal byproducts. But I don't consider myself vegeterian and never will.
Edit: I tend to look at this like a religion. Can someone be Christian if they aren't in church every time the door is open? Do they have to follow every rule and think correctly and so forth 100% of the time? I think most people would say no.
Well, how long must you have been one to actually count as one? If you last ate meat 12 months ago, are you a vegetarian today? Or does it have to be five years ago? Ten?
Depending on how we set this cutoff, we get more or fewer "vegetarians".
How about if we look into the future? If you're one week from relapsing into meat eating, are you a vegetarian today? What if you're five months away from downing a hot dog? Or how about five years away from enjoying a juicy, char-broiled steak?