In the shadows, away from UV exposure, you can check the progress of your image exposing.
Take off the posterboard and gently peel back the image. DO NOT completely remove the image unless the exposure is complete.
The exposure is complete when you can see the linework burned into your screen with the positive peeled back. As you can see in the image on the left, there's a clear division line where the paper was. We want to see that but with the image itself. I exposed this screen for another 10 seconds before rinsing it.