Realistically, however long it took for someone else to notice you were doing it and call them.
Source: had the cops called on my friends and I multiple times for having foam sword fights in parking lots at college. Apparently people from a distance thought actual fighting was going on. Not sure if that’s a testament to our acting or their poor eye sight.
I love eldricth horror precisely because of this. Imagination will almost always be scarier than something that can be put into words. Descriptions give handles to hold onto for your understanding, boundaries and walls for the horror to fit in.
Give me more vagueness about how, gazing at it, the room could not have possibly contained its size. The feeling of the split second while tripping before you connect with the ground, stretched into an interminable constant in the back of your mind.