The epsom salt is to clean the intestine of the fish and improve the appetite and once the fish starts to poo you stop the epsom salt and as you do water change it gradually disappear fro your system , your water change is depends on how big is your tank ,how many fish you have and whether you have adult or juvie discus , but as a general rule i say 50% w/c twice a week.
I strongly suggest the you study the stickies in the " discus for beginners " in the forum section and get yourself familiar with the basic needs of discus .
HTH