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.