In my head I have a Johns Supes omni mapped as collecting Up, Up, and Away, his Action Comics run up to the end of Brainiac plus annuals, the Legion of 3 worlds mini, and his Adventure Comics issues as well. I think that comes to roughly 1200 pages.
I'm leaving out his last four Action Comics issues as they were part of the New Krypton introductory crossover, and I can't remember how well they read on their own. I'm not including the full crossover, because I'd prefer to leave space for the Legion and Adventure Comics issues, which pick up some threads from the LOSH arc and other Johns stories. Plus, the NK issues would fit better in a hypothetical New Krypton omni.
I haven't read Busiek's run yet (aside from Up, Up...), but it is somewhere on my ever growing to read list!