Fast, there's not really any lag, so it's down to how long your opponent spends thinking. I've had straightforward games last under a minute. Average seems like it's around 1.5-2 minutes lately for me. I mean if someone brings a deckout stall you're still gonna be there a while, but there's basically no time added by the game itself - it's pretty much entirely however long you and your opponent take to make their moves.
Edit-if you want to get a feel for it, ask for a pvp match in in-game chat or discord. People are around pretty frequently right now so decent odds you’ll be able to try it out.