Two thoughts:

1. I don't know if same-day (4-hour) confirmed change updates the PNR at all. The flight is already over in the departure management system, which is the only one that really needs to know your change. I do know that when you call to do the 4-hour confirmed change, the e-ticket is not revalidated, so you can't use OLCI or kiosk check-in. So possible "never" is the answer to "how long does it take to show up on ual.com".

2. I was surprised today to find that a change made on the phone was immediately visible in my ual.com session, even though I had not logged out and logged back in - in fact the agent had just rebooked me and I did a seat change before she ended the call. Just had to click on "My Itineraries" again and the change was there - never happened like this before. This was not a same-day confirmed change, it was a change for a few days in the future. They may have updated how info flows from the real PNR system to the web front end. Or maybe I just had a day when ual.com wasn't being united.bomb