20 chapters is the hard cap. Can't see this arc going over 20 chapters, especially since Oda wants to move on to the next island (probably Elbaf) and start some major clash (possibly Shanks vs Blackbeard) this year. Alabasta and Skypiea were both in the 60-70 chapter range, so Egghead probably won't be longer than that.
The bulk of the rest of the arc is probably going to be Vegapunk's announcement and fighting the Marines to escape. Somewhere around 12 chapters is reasonable, with a bit of an epilogue after to set up for the next arc