If you are travelling solo, you can request that your upgrade request be considered as late as at the gate if not confirmed earlier. Say someone doesn't rock up or they have a missed connection then they would have to wait till they know they won't make it before upgrading someone.
For international the defined benefit is up to 48 hours out as a WP, but in practice it can be more than that, perhaps up to 60 hours. Note if it's a route with Chauffeur Drive those bookings close 48 hours out, so if you get an upgrade to J before 48 hours out jump online and book your Chauffeur Drive ASAP.
I haven't had an upgrade at the gate yet, but I have been in lounges and have heard people being called to the front desk. I think a fair bit of the time that would be to give them a new boarding pass as they've been upgraded.
I had a friend who got upgraded at the gate for the DXB-LHR leg of QF9 as a SG. Upgraded that late my friend had no choice of seat. My friend got a seat in Emerald City while I was back in the main Business cabin.