I don't think there's a set amount of time, but I just hooked them up to my computer, put winamp on random, and let them play for about 3 or 4 days. I'd say put the volume on louder than you would listen to, but not loud enough where it will damage the drivers (you don't want them clipping). After that, I put a song on repeat called 'bass test' taht is all low frequency bass, because that should loosen up the drivers the most. I did this for a night or two. The validity of all this is up for debate, but I see no harm in it. Most burning in is psychological, imo, so just listening to them will help. I can send you that bass song, if you like; just pm me.
And yes, the md33's are far far better than the stock buds. I was impressed with them.