I usually cement, wait about 10 minutes, blow through the tube for a bit, and then run tap water through it. That'll make sure that the cement on the inside of the pipes is cured on the surface, although the rest of it might take a bit longer since it is sealed off from the air. A good way to figure it out is to touch the ring excess cement will form on the outside of the pipe where you push the pieces together. Once it stops imprinting your finger print, it's good to go. However, if you can wait for 24 hours, wait.