It's three years since I last went to the cinema.
It's been three years since I last went to the cinema.
I chose It's been because I think this is an action which started in the past and continues in the present. Moreover, since usually goes with Present Perfect. This maybe my confusion, but whenever I see since, I automatically get Present Perfect.
However, my teacher got It's as there's a formula like It is + time + since. But I got very confused as I don't know when to use which?
As I searched in https://dictionary.cambridge.org/grammar/british-grammar/linking-words-and-expressions/since, it seems that both is correct. But which would you as a native speaker often use?