Usually it just takes minutes. If you are using the primer/glue combo, you'll notice a pretty strong smell for a while. I'd wait till the the smell subsides before running it on an existing tank. Also might want to run tap water through to rinse any extra residue before attaching to system. I'm always freaked about poisoning my fish, so my suggestions may be a bit on the overly concerned side.
You won't be able to change after a few seconds after you put two piece together (working time), but it will take a day or two to let the VOC (volatile Organic Compounds) to gas off (curing time).
