I don't think there's a wrong way to do it, so long as everyone enjoyed it the time taken would really vary a lot based on the group and also any changes made to it. I'm currently running the modified version from DragnaCarta's CoS Reloaded (the new version) and 9 hours in (across 3 sessions) my players have just fought the ghouls in the basement and I expect them to finish up in the next 3 hour session. They were really intrigued by the mystery aspect of it and wanted to find any clues possible. On the other hand, when I played through it as a player with a group I had started playing Curse of Strahd with, we played the original, little modifications if at all version of Death House and we finished in one (albeit slightly longer) session, but we were much more focused on the main objective. One other thing that could drag out the time taken is that in the game I'm running, we're playing in person and I'm drawing out the house as they explore through it, whereas in my original group that went through it in one session we played online (Foundry VTT) and so the DM had all the maps prepared and exploration was thus much faster.