I just got into the game myself, and I have been playing exclusively in the desktop app. I've bought around six or seven booster packs in order to be able to scan cards in to my online library. I've noticed a few times where the wait to play a casual game is more than 30 seconds, but most of the time it's shorter. I don't know if this is because of the time I'm usually playing or not... I'm usually playing in the afternoons EST time zone. At the same time, around half of my time in the game is spent doing all the single player campaigns, since that earns you gold coins, which
can be used to purchase online booster packs. Basically I have not had an issue yet finding a game, but your mileage may vary.
EDIT: I have about 46hrs total in the game. Not much, but not bad for having the game for two weeks.