Bucket and upgrade-mode jargon aside, in my experience in the past year, the most common time for an upgrade is about 3 hours before the flight. I know this because I forward the CO upgrade email alerts to my cell phone, which makes a special sound when they arrives.

For a 752 (BF seating) IAH to SFO last night, I got my EAU about 5 days out, planning to give it to my wife. Checking in for our first leg at JAX, I made sure to put her on the companion upgrade list for the IAH to SFO leg. This was about 6 hours out, and the upgrade cleared immediately, so we both got the best seats in the domestic system.

Others can supply appropriate jargon here, but this was exactly the outcome I expected. A Plat companion upgrade is normally a catch-22, because one seldom checks in more than three hours in advance, at which time Golds will already have been upgraded into the available slots, leaving the companion only with battlefield upgrade prospects. However, on trips with connections, one can play it as we did, snagging the companion upgrade.

I'd separate unnecessary worrying about upgrades from tactics. Worrying is pointless. The above tactic, however, is generally effective; if one lived close enough to the airport it could be actually worth a separate trip that day to check in, to snag a transcon companion upgrade.