It depends on his fitness level. If he is very fit, it might take him around 10 - 15 minutes.
He could be doing it for a bet. Perhaps he wagered with his friends that he could climb that many stories.
It could be that he is in an emergency situation. For example, there was a fire in the building and the stairs were blocked, so he had no choice but to climb the outside of the building. And he might have some basic climbing knowledge and enough courage to attempt this dangerous climb of 15 stories.
He might be sweaty and a bit out of breath from the climb.
If he was climbing for a legitimate reason, like a rescue operation or if he had prior permission from the building owner, then it could be legal. But just randomly climbing 15 stories of a building in Philadelphia without any authorization is likely against the law.
It really depends on the writer. A very fast writer who can produce 1000 words a day could finish it in about 112 days. But that's not very realistic as most writers need time to plan, revise, etc.
He might have used the external structures of the building, like window ledges and drainpipes, to pull himself up.
It really depends on your reading speed. If you're a fast reader, it might take a few weeks of dedicated reading. But if you read more slowly or take breaks, it could take months.
It could be that the elevators are out of order and he's in a hurry to get out of the building, so he chooses to climb down 15 stories. There might be an important meeting or appointment he doesn't want to be late for. Also, perhaps he just wants some adventure or a physical challenge, although climbing down 15 stories is quite a risky adventure. He might be a thrill - seeker who likes to do extreme things.
If she's a fast reader and devotes a lot of time to it, she might finish it in a day or two.
It could be a dare or a part of some extreme fitness challenge he set for himself. Some people like to push their physical limits in unique ways, and climbing 15 stories is no easy feat. It might be his way of proving his strength and endurance to himself or others.