This seems to work for all non-combat pets, and is not restricted by any time duration. A player may be logged out for any number of hours or days, and as long as they had a pet summoned prior to logging out, that same companion will be there upon log in. Talk about "through thick and thin!" Or in this case, "through log out and log in."
Working As Intended?
Although this may be a neat feature on the PTR, we aren't certain if this is a bug or that this change to vanity pets will go live when patch 4.1 comes out. It's definitely something to be on the look out for in the future, though!