Including pitchers batting, the average OL team has 6,423 PA in a season...
So, if you subtract pitchers, I'd estimate somewhere around 5,823...
Then if you subtract the 10%, you'd need about 5,295...
Then if you figure you'll get some useable AAA (i.e., worth about 75 PA each, times 6), you don't have any reason to ever draft more than the minimum of 4,800 for an OL (unless you're drafting a heavy offense team in a strong hitters park or plan on using the WW)...