About 2 hours probably.
I'm going to answer this from the perspective of one of those spoiled kids who gets an excessive amount of gifts.
We are in our 30's and still get close to 80 each (me and my brother). DB doesn't get as many now, as my parents mostly give to his kids instead now. Takes about 2 1/2 hours.
Last year was DH first Christmas with us and so it took longer because he was not used to it. DH has a hard time with this.

I grew up very well off and he grew up in poverty. He feels guilty and overwhelmed but there's no arguing with my parents. Last year, my parents and they got him their usual standard 80 or so gifts.
This year, my parents have decided to give DH & I and DB & his family each WDW trips with a budget of $4K each instead of a "big" gift. We are also getting about 25-35 or so gifts to open and I hope DH is going to be okay. I know they got DH a level one "Rosetta Stone" language program as well as some jeans, about 15 video games (mostly vintage ones for the NES, but I think there was 2 new ones) , PS3 Move, a couple of t-shirts, some team logo stuff for the Bruins and a bunch of gift cards to gamestop, etc. I have no idea what I'm getting.
We made lists because my mom demands lists. I have tried to explain to DH that if you don't make a list you're still going to get as many items, they will just more than likely be in the form of gift cards and things that you don't actually want or need.
I usually don't talk about it. But I'm replying here because some people seemed really shocked at that kid who gets 40+ gifts. I hope she has an easier time than we do. I don't know if it's jealousy or what, but people who DO find out about how generous my parents are typically roll their eyes and are just generally pretty horrible to me and DH about it. It's annoying and hurtful. One of DH friends found out last year because he saw the new (huge) stack of PS3 games and he's made a few rather snide comments this year. It comes off as petty and childish on his part, IMO.