This depends on what you define as in game. According to Steam, I’ve spent the most time on NGU Idle at 3306 hours, but a large amount of that was spent with the game running while I was asleep or at work. For Steam games with the most hours, I’ve spent 840 playing Path of Exile but that doesn’t include the 200 or so hours playing the standalone client. This is followed up by 440 hours playing Terraria (which doesn’t include console version hours), and Realm of the Mad God which I have 325 hours playing.
If we include MMOs in the mix, I’ve easily clocked 1500 hours playing both World of Warcraft (between Classic and Retail up to and including Mists of Pandaria) and Guild Wars 2 over the years.
Funny enough, I wouldn’t include any of these games in a list of my favorite games other then Terraria, so that’s good to keep in mind :)
I noticed at the start of the final act that my save folder had ballooned to 4.2 GB which seemed a bit outrageous to me. After a bit of digging, I found that for some reason if you play on 2 separate machines (steam deck and a desktop PC for example), the game was failing to replace your old saves with the new ones when switching, rather syncing the new saves made while leaving the old ones in place.
After manually deleting the saves and setting quick and auto save limits at 5, cloud syncing is infinitely faster due to a vastly reduced size. You do have to go in and delete all your old saves each time you swap platforms unfortunately, but this is at least a work around for those like myself who don’t want to feel stuck only playing on one platform. It also makes the cloud syncing (at least through steam, I have the in game cloud sync option disabled) far less time consuming.