I put 30 hours in in total, did the campaign about 10x probably, I really enjoyed unlocking the shop items and then blowing the shit out of everything on the harder difficulties that I wouldn't have done normally, or at least I would've done it once and then stopped. I think my best time was 1:08 but that was with infinite... everything lol. Legit best time with no shop items was 1:35 on hardcore.
It was super fun to run through it many times for me because it was like a rollercoaster ride, looked amazing in 4K60 as well, I also did my final run at 6K30 (downsampled to 4K) which looked amazeballs, certain scenes were like concept art.
I paid £32/$42 for it a week or so after release but would've been fine with £45/$60 given the value I got out of it.
If you want to try out the shop weapons without having to run through the campaign a billion times then just load a save (Or you can get back to that part in about 45 mins anyway) at the hospital hold out scene. When you get to the bit where the cutscene of all the zombies coming through the door then just kill the 10 or so in front of you (With a grenade or the assault rifle) then immediately reload the checkpoint (or you can kill another 20 or so that come through the door but reloading after killing the "group" is the fastest) and do it again, until you have killed 2000 zombies lol.
I thought it would be awful but it was pretty fun blasting them in all the different ways you can and it took 45 mins max to get 2000 kills. With that "money" you can buy all the infinite ammo guns and have fun blasting everything.
edit - oh yeah and my first playthrough was 10 hours, but I'm a slowpoke.