As I said it's pretty much market (and time of day) dependent. In the EWR-FLorida or EWRLAS markets it will rarely happen that all seats wil go on the 24hr EUA.

This is the fundamental design flaw with the EUA system. It's not a waitlist, so inventory appears, and others can snatch it up using other mechanisms (e.g., checkin, mileage, etc.).

CO really needs to make this a waitlist, and the instant inventory is available, it goes according to waitlist priority.

It's not a flaw, it's intentional. Why would anyone want to give a product away for free when there is a opportunity to sell it?

One poster noted that seats become available and disappear in the F cabin before departure, but EUA's may or may not run. That is of course, CO's business choice to open the seats up for upgrade. As frustrating as it may be to see EUA's not run at 3 hours or at other times if you are in a lower class fare, it is even more frustrating if you purchased a Y fare and no upgrade was available at time of purchase and you don't receive upgrade when seats open up. As we all know, EUA's do not run continuously.

While CO flyers may not be aware of Expert Flyer, AA elites, especially those who fly long international flights do use this service. Among the various services available is a notification service that a seat has become available in a cabin or a specific fare class on a specified flight on a carrier. Once you setup for an alert, Expert Flyer will periodically search your selected flight and send you an email message if a certain fare level or certain cabin has become available.

I have also used Expert Flyer on CO to snag an F seat when I am on a Y fare and there was no first class inventory at time of purchase. As previous posters noted, inventory becomes available in F in the days before departure. At least one time, I have used automated alerts on Expert Flyer to inform me that a seat in the F cabin has become available in order to snag a seat for my YUP fare by calling the Plat line. While presumably a Y fare holder would get the upgrade eventually when EUA's ran, there is also the possibility that someone else could come in later but before EUA's ran and purchase her own YUP fare and deny the prior purchaser the upgrade. In this case, it was nice to snag the seat before the EUA ran.

Last night, PBI-EWR at 8:05pm... Ms. TommyC80, as a Silver, was not upgraded at T-3. She was #1 on the standby list and there were 15 unsold seats in F. It looks like the gate agent upgraded her around 6:30pm...which seems really early. FC ended up going out with 12 empty seats.

I was upgraded at T-5days.