> If I am vegan for January and then in February eat meat, am I retroactively not a vegan for January?

Like many labels I think it comes down to intent more than timeframe.

If you go vegan and then a day/month/year/decade later you change your mind and decide it's not working then you were vegan during that time and are no longer vegan.

But if you take a hiatus from consuming animal products with intent to go back to eating meat in a few hours/weeks/months then I wouldn't say you were a vegan during that time. If I happen to go a few days between consuming animal products that doesn't make me a vegan during those days.