A little over halfway through the story, Tom has a revelation when he looks into his living room, and it's this: even though he feels like he's been outside on the ledge of the building for a long time, it's actually only been a few minutes.
He can tell because he sees "a thin curl of blue smoke" on his desk, and he's astonished to realize that it's his own still-lit cigarette that he had left there.
For Tom, this revelation is "past all belief." His movements outward to get the paper, and back toward the window to make his return to safety, seemed to have taken a very long time; events that truly happen in an instant seem to stretch out for Tom into painfully long moments.
Tom's realization about how long he's actually been outside the building has important consequences for this story, both plot-wise and thematically. It precedes his later revelation that his wife, Clare, won't be home to rescue him for several hours at least, and it's this second realization that prompts Tom to try his hardest to break the window and get back in. Thematically, Tom has to struggle with the concept of time, and he has to realize that he's been wrong about how he understands the nature of time and how he's been squandering his own time in life. The revelation about how long he's been outside, and the follow-up realization about how long it will be until Clare comes home, help prepare Tom for his final epiphany: that he's been wasting his time on work projects that don't matter, and that he needs to spend his time more wisely with the wife he dearly loves.