Walter, Olivia and Peter spent nearly an hour inside the pocket universe. Based on the man's claim he had been stuck there for five days while 20 years passed in the normal universe, each hour in the pocket universe would equal roughly 60 days in the normal universe. Astrid would have been waiting for 2 months for them to return.
The apartment was apparently blown up and abandoned 20 years earlier, yet there is a functioning light on when they enter.