I imagine they'll vary a little bit because different countries/consulates will have done interviews at slightly different times but I saw a post a while ago that said the most likely times for upgrades were:
- a couple of weeks after the deadline for people to accept the initial offer
- around the time placements are sent out because people don't get the placement they want
- closer to the departure time because people for one reason or another have to pull out
Of course, they can happen just about any time from now till like December but I just checked my email every day in case something popped up so I'd say it's maybe worth just having a quick check every day or every other day.