Well it depends. I created 1st person adventure games for 10 years, and giving a correct playtime is always a challenge for me :D I played your game quite slowly because I needed time to find the solutions to the puzzles. I'm not a beginner with puzzles, yet I got stuck here and there in your game, so I would say it is not on the easy side (not super hard either). That would make sense to increase the playtime, but I also see that you have uploaded a playthrough video. If the playthrough is a kind of walkthrough that gives all the answers, then it means that some people will not do the effort of finding all the answers on their own, and so they can end the game very quickly, in 20 minutes or less. It is probably better to write an average time. 30 minutes sound fine to me. In fact it doesn't really matter, only enjoying the game is important, and I did!