they fixed quick battle pet leveling using tamers... you used to be able to do 3 rounds with a tamer and level from 19 to 25 on a single pet... each win would net you like 2 levels... by simply tunneling all the xp to the non 25 in the group and letting them kill the 2 25's... rinse and repeat.....
i started pet battling about a week ago... got my first set of 25's. They took me (15-20 hrs) Then i did all the pet tamers... (had to open a ticket to get my safari hat read about it in last patch notes). I started the pet battles right after the last patch so i got 3k Thursday when finished the last tamer.
then figured out this method, I was pretty sure it wasn't as intended but I actually did try to google it and couldn't find anything on it... and leveled 41 25's... (38 of them are rare wild pets captured in pandaria... lots of critters, beasts, and aqua so only needed to level 2 levels)
Yesterday... after a whole week of intense pet battling.... I log in and the magic is gone... I like the collecting.... i'll probably finish getting all my rares... I've probably collected 3/4 of the wild pets as rares.... I'll probably finish turning all of my non wild pets into rares... but i'll not be leveling all my 450+ pets to 25's. it's just not worth that now.
I can't say for sure how long it would have taken to level them all up... maybe I could take a low level to 25 in an average of 30mins each before the change...so the remaining ones would have took ~205 hrs (8 days) of game play..... now it's like 5 hours each....about 2050 hrs (85 days)