This is a text-book example of the "no true Scotsman" fallacy.
Many vegans/vegetarians would accept you to their "club" only after being on probation for a year or two.
It's analogous to saying "most martial artists quit in less than a year". For some definition of "martial artist" that may well be true, but it wouldn't be a definition acceptable to those who actually practice.
So, gatekeeping instead? "You're not a real X unless Y," where "Y" changes to keep out the undesirables.
"You're not a real programmer unless you can do Dijkstra's on a whiteboard from memory."
"You're not a real vegetarian unless you don't eat meat for at least as long as I have."
You will not be accepted as a programmer until you've written some code; or as an athlete until you've done some nontrivial athletic stuff. Why should othe categories be different?
From what I understand, many people may make a religious vow or commitment to eat vegetarian for a specific period of time (for good luck or out of gratitude for a benefit they received) and then commonly eat meat again after that period of time has expired. I wouldn't be inclined to say that these people "are vegetarians" in my western sense, any more than I would say Eastern Orthodox Christians "are vegetarians" when meeting them during their fasting days or seasons.
What difference does it make if someone who's been a vegan for 3 months calls themself a vegan? Are they "harming the movement" by doing so? If partaking in the diet doesn't require you to subscribe to the ethics, as you say, then the label should just be a label: someone who doesn't eat animal products. No time frame or reasoning attached.
I honestly think it's reasonable to attach a minimum time frame to a word like this, otherwise it gets a bit hard to know what people mean when they speak. Whether one year is a good choice I don't know, but a minimum is required.
Can you expand on this? If there are two people in front of you, one who has been a vegan for 20 years, the other for 2 weeks, what is the real difference between them, how does that time difference change your interaction with them?
To put it another way, why does one word have to apply to one and not the other because of some inherent time requirement? English, in particular, as a language is inherently vague, and we rarely make this time requirement distinction for other words that describe people.
If I just graduated from the police academy I'm a "cop", just like how the 30 year veteran detective sergeant is a "cop". No, we aren't the same, since the latter has far more experience, but that doesn't mean that only one deserves the title. It just means we use additional qualifiers ("30 year veteran detective sergeant") to elaborate on what type of cop he or she is.
Personally I'd set a minimum of a year.
I'm being pedantic on purpose here because the idea of some minimum time requirement to call yourself a vegan is one of the most pedantic things I've ever heard, and supports the stereotypical characterization of vegans being overall pains in the ass.
I wonder what proportion of the cohort here were in each group.
That doesn't apply here.
We're starting and ending with the same definition.