I typically list between 100 & 500 items at a time. In GS6, verifying 500 items might take a while, but never more than 20 minutes or so. In GS 7.0.3, I am getting a beachball whenever I try to “start” more than about 100 items, everything slows to a crawl, UI events no longer update, and in many cases the verification process kills other scheduled listings, apparently by interfering with image ftp.
When I have managed to verify 100+ items all at once, it has taken more than an hour. I cannot imagine what 500 items would take, because I have to Force Quit the app because it is so unresponsive when I try.