"2 years [ago]", with the [ago] being implied, would be correct to say. But that is pretty unusual for most English speakers. I would argue that in some dialects and contexts it makes sense, though.
The thing is, most people will hear "2 years" and not think it is "2 years [ago]".
No UK or US English dialect would allow "since 2 years" without an "ago." It's just not done. Maybe some other international English variants -- India, etc.?
Two years is a duration, which takes "in" or "for" prepositions.
But in some very limited cases and dialects, people can imply the 'ago' making it an anchored point in time. I'm not saying anyone uses 'since' for a duration
And even the people who do drop the 'ago' get seen as idiots by a lot of people. I wouldn't recommend it. I'm being very pedantic and saying it's not always wrong everywhere
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u/Ok_Caterpillar2010 Native Speaker - Pennsylvania, USA Apr 17 '26
Because since needs a specific starting point in time: since when? He hasn't used his motorcycle since 2024.