At first he notices a few minutes have passed, and he spent about 20 minutes talking to the girl.
Then he tests and notices 1 minute outside is 2 hours inside.
Then she says 1 hour inside is 6 days outside.
Three inconsistencies right off the bat. That's how good this manga is.
The first time he noticed the discrepancy, it would've been seconds off from the watch to the phone.
And when he tests, 60 minutes inside is 120 hours on the outside, so 5 days.
Author can't even be bothered to math for his story.