I don't think he could have been there for that long. All of his speech is in direct dialogue. It would not take an hour and a half to read everything they both said out loud.

There is really no narration to account for any passage of time beyond what they were saying out loud except these little bits :

"A short pause of mutual thoughtfulness succeeded."

"From a reverie of this kind she was recalled at the end of some minutes by Willoughby, who, rousing himself from a reverie at least equally painful, started up in preparation for going"

I don't think these could amount to a full hour of time.

It's very possible that Brandon and Mrs. Dashwood got there sooner than the colonel had calculated. Mrs. Dashwood was ready to go when he got to the cottage, so that must have saved half an hour or more. I also don't think that 10 p.m. has to be the time Brandon gave her. She thinks 8 p.m. would be 'almost' impossibly early, but if it were 10 p.m., she would be certain it was them.