At least 50. I didn't expect Egghead to shape up to be the fourth longest arc but that happened, so with Elbaph I expect Oda to take his time. Especially with the pace he's set so far, he doesn't seem in a rush. Things are heating up but I doubt the confrontations on Elbaph won't be important on their own. I expect they'll have a certain importance and we'll get a decent amount of the Holy Knights, Loki, the silhouette man and other mysteries and reveals then it'll tie into the next arcs to come. I said 51-70. I think Oda can handle it in the 50's. I wouldn't mind it being less than 50 though, we'll see.