I would wait 15-30 seconds (about the same as boot up, in most cases) but it might be a good thing to poke a hole, to see it. Another option (on my 3D printed cases when I first started) I used a piece of thick fishing line superglued to the activity LED and fed out an opening (like a light pipe)...https://www.youtube.com/watch?v=J22fCKgOAYs
