It says to let it cure for at least 24 hours, however, I haven't had a problem with letting it dry for 15 to 20 minutes and painting on it. The images below are a watercolor using a stamped image and Dr. Ph Martin hydrus watercolors. There is a slight crinkle, but after it drys you can hardly tell. Disclaimer: I am a beginning watercolor painter. So don't judge the materials from my paintings:)
The second picture with the balloon is the back side of the painting. You can see a little spot where I went outside of the watercolor ground.