I personally can go like 2 updates ahead tops, MAYBE 3 or 4 if I'm in a plot point that I am incredibly excited about and most of the pages happen to be easier ones layout wise. And in the latter case the 3rd/4th ones just end up being lines to pick up later on.
That's like 2/4 weeks ahead depending on what comic has the buffer. (one is weekly, the other is Every Other Week)
And then other times I just barely manage to get the update out before the day it's supposed to get out.