CYA does not dissolve readily in water, so it can take a while to fully dissolve. How long depends on water temperature and water flow around the particles. But based on my own personal experience and others who have posted here, a week should be plenty.

With you CYA at 50 ppm (when it eventually gets there) you will need to keep you FC at between 3 and 6 ppm ALL the time to keep a good level of sanitizing chlorine. Here is a post that gives the specifics:
http://www.poolforum.com/pf2/showthread.php?t=365