I had first seen this on Star Wars Episode 1: Racer (1999) for the N64, check this video at 10:36 : https://www.youtube.com/watch?v=CSvXIAnVzYE&t=10m36s
36 days suggests you're feeding photoscan more than it's designed to handle. I'm not familiar with it but surely there's a way or either breaking it up into smaller chunks or settling for lower fidelity?