Usually its only 1 month between releases. However Miura has been known to take 2-3 months in between at times due to certain chapters just taking that long to draw. He does every single square centimeter of Berserk by himself and by hand. This leads to extremely masterful and detailed artwork, but also insanely long waits between chapters at times.
If you look at the latest chapter for example you can see that the very last 3 pages must have taken him forever to complete. Nearly every single one of those soldiers that is close enough for even the slightest amount of detail is unique in at least 2 or 3 ways.
