Short answer is forever.

Long answer is it depends upon your case. Obviously you haven’t done much research on the subject before posting your quesiton, otherwise you would have mentioned what type of process you are going through (EB1, EB2, EB3 etc) and what is your Priority date, LC date.  So, with that limited amount of information I will just tell you to expect anywhere from 1-4 years before you will see that green thing.