How long had it been since Clare left? How long would it be before she would get back?
Time seems to have slowed down for Tom while he's out on the ledge, in terrible danger, and he realizes that his wife Clare has only been gone from the apartment for about eight minutes. Further, it'll be at least three hours and maybe even four before she comes back, since she went to see two movies in a row. This means that Tom will have to hold his balance, endure the cold, and try to stay conscious for many more hours before being rescued. He realizes that he'll have to try to break his way into the window instead, because he can't last that long.
We'll find all these details about two-thirds of the way into the story:
He couldn't possibly wait here till Clare came home. It was the second feature she'd wanted to see, and she'd left in time to see the first. She'd be another three hours or--He glanced at his watch: Clare had been gone eight minutes. It wasn't possible, but only eight minutes ago he had kissed his wife good-by. She wasn't even at the theater yet!
It would be four hours before she could possibly be home...
As you can see, Tom's experiences on the outer ledge of the apartment building have felt like much longer than eight minutes, but that's all the time that really has elapsed. Just before he suddenly understands what this passage of time means for himself and his situation, he's actually somewhat relieved to have gained a relatively safe position near the window, and he finds it kind of funny that Clare might find him there when she gets home. But this realization of Tom's about how much time has actually gone by really sobers up his thoughts and adds tension to the already highly suspenseful story.