I remember the actual play being excruciating, over three hours long, but I believe that written down it's rather short. It's often just waiting, with long pauses and empty stares.

Uh, it’s not. I suppose people can produce a given play however they wish but I don’t think three hours is typical…

Web searches suggest 2.5–3 hours is typical, both for stage productions and film adaptations.

A 2021 Broadway production was 3 hours long. A 2013 Broadway production was 2.5 hours (including intermission). I couldn't find any shorter examples.

Some of Beckett’s other plays are much shorter.

Ah? It should last around 2 hours, usually (like to what your 2.5 hours with interval comes down). I don't remember anyone complaining about the length when we went to see it in high-school; which speaks for itself, I guess, eh :-). The French performances (which are relevant since it was written in French) I can find last between 1h45 and 2h10. Perhaps in the US, like for TV programs, they inserted a total of 45 minutes of commercial breaks ;-)

Plays usually have a ~15m intermission.