“The Mechaber says that, but Mishnah Berurah says many Acharonim disagree.”
How do you see that from the Mishnah Berurah you quoted?
All he said is that if you went to the bathroom and didn’t say Asher Yotzar until the next time you went to the bathroom, you would only say it once rather than twice.
There’s still no specific time window.