I watched the upgrade list on the flight status page for the first three months of this year for the flights that I had booked. I focused on one route that I fly most frequently. The following data are for 19 flights.

14 upgrades out of 160 available F seats at T-120.
36 upgrades out of 208 available F seats at T-96.
55 upgrades out of 264 available F seats at T-72.
82 upgrades out of 316 available F seats at T-48.
139 upgrades out of 336 available F seats at T-24.
148 upgrades out of 336 available F seats at boarding.

I didn't always start checking at T-120, which is why the number of available seats is lower at the beginning.

I didn't always check when the flight was closed out. When I stopped checking, there were still a grand total of 28 F seats available, which means there could have been a total of 176 upgrades.

(Before you start guessing how many people are 1K, Plat, and so on, it is important to understand that the CPU process doesn't always run. There have been several occasions where no one got upgraded between T-120 and T-24.)

The numbers suggest that UA gives away between 44% and 52% of all F seats to CPUs and RPUs (and whatever else might get you on the upgrade list).

So, what about TODs? Or more accurately, what about people who purchase an upgrade during the check-in process? The grand total of F seats that were booked without an upgrade after T-24 is ... -1. Well, that doesn't include the 28 seats that were unaccounted for, so the number could be as high as 27. Even at that extreme, that's still only 8% of all available F seats.

For what it's worth, out of these 19 flights, I flew 18 of them, and I got upgraded 14 times.

Life is good.