Originally Posted by Zorak

72 hours is the earliest that you *could* clear, but you won't necessarily get the upgrade just because there is an open seat. Upgrades are inventory-controlled (from a separate OU fare bucket) based on how many F seats revenue management thinks it can sell in the remaining days before departure.

If you clear automatically there will eventually be an email, but you can indirectly check for inventory yourself by pulling up the seat map. If you see a buy-up offer* then there's no inventory. If the buy-up offer disappears but you can still see F seats in the seat map, you can try picking one -- if it says FREE you can confirm the new seat and self-upgrade, essentially. (If this state occurs, it ought to be the case that you would have received the upgrade in due course of time even if you hadn't done anything, but I prefer not to leave things up to chance, or Delta's systems...) You'll still get the email eventually, congratulating you on being upgraded to the seat you chose

*aka "FCM" -- other related thread: First Class Monetization, or FCM: The Definitive Thread