for me it only took  like a week but i heal fast  i think they say to wait a month though

 Wait until all the swelling goes down if possible... If not then go to your piercer and have them change it in steps! 

My piercer said wait two weeks then come back. He changed it for me, allthought the swelling had gone down after a few days.

 There seem to be two stages to swelling, the intial swelling and the residual swelling... I had to downsize my bar like 3 times until I got a proper fit... The intial swelling goes away pretty fast, but there will be a "ring" of swelling directly around the barbell which takes a while to go away... But changing the jewelry to often can result in the swelling to come back... that is why its best to wait it out unless the longer bar is interfering with eating and talking too much to handle... 

 I down sized to an inch bar after the initial piercing at 1.5 inches (If I am recalling correctly).  I stayed there until recently when I realized that I had far too much play in my bar.. Now I wear 1/2" bars and love them.