Hi Tuskat,
The less unexpected roadblocks there is for the players, the closer the playtime estimate should be.
I've completed the game after about 45 minutes and now that I know what to do, 10-15 minutes is around the time it would take me to complete it again. I think to give better playtime estimates, you have to get better at finding unclear elements of your game(s). For example, there's two important things that could be communicated better: 1) You have to use the environment to kill most of the enemies 2) A button stays activated if anything touches it. Once I understood these two things, the game became a lot clearer/easier.
I know there's some text that hints at the first point so maybe I'm just slow, but I still think it could be better. One reason I think I was confused, is that the first time you encounter an enemy you are required to kill it with your weapon without the environment. Also why would there be a shotgun upgrade if I'm not really supposed to use my gun? Why is there even a gun? It seems like a crazy question, but if the game is focused on using the environment, why not go all in! If I was forced to use the environment, I would have had no room for confusion. Anyway, all I'm saying is the design could be tighter.
About my second point, I think just changing the button's sprite or color when it's pressed could be enough. Generally I find it's better if things change visually according to their state.
Finding the unclarities in your work is definitely a difficult task; I know I always leave some in my games and it's mostly what gets people stuck (that and difficulty spikes). You will get better with experience though. Getting feedback from people is also a big help. I don't know exactly what your friends told you, but maybe you could ask them to be more specific? I find you really don't need many testers to find the most obvious problems.