This is my experience over 5 years flying from Ohare.
Direction, Time, Season, and seat class all have their part to play and can substantially change the outcome if all align perfectly, but if you are the type of business traveler that has no set territory or direction like I do then this is what to expect.

Silver - 1/20 flights get upgraded

Gold - 1/10 flights get upgraded

Platinum - 1/5 flights get upgraded

1K - 1/2 flights get upgraded

Cleared UA519 for Sunday, 8/25 on a 739 as a gold on a K fare at the CPU window, T-48. Seemed 1 GS was upgraded, 1 plat, and 10 golds, all at their respective windows.

I've had two this year so far as a Silver (usually on a G or K fare). The first was a Memorial Day red eye SEA-ORD, the second was today, 6:30 am flight ORD-SFO. Both times cleared with an email at T-24. There were only about 60 people on the flight this morning and first went out with empty seats.
These flights are probably good CPU bets in general, since if I clear, pretty much everyone clears :D

Cleared the SEA-ORD red-eye tonight on UA 1993 as a silver on K fare. Definitely doable!

Congrats!! The last time I took the SEA-ORD red-eye my RPU didn't even clear as a 1K. It was also a Friday night which I suspect has a higher load. Also, per usual, I remember a ton of GS pax boarded.

My wife (no status, but flying with me) and I (silver) both cleared at T-24 for DEN-SFO tomorrow morning. The advantage of flying at 5:30 AM! The flight looks less than half full.

Cleared ORD-SEA early am at T-36 and the return SEA-ORD red-eye at T-47 as a Gold this week. Maybe a lower travel week with the holiday next week?

Cleared LAS-IAD on 8/15/17 at the gate. Last seat remaining. 1k on an S fare (originally P, had to change flight)

If you had a P fare to begin with, I don't think this counts as a CPU, unless you got a refund on the fare difference between P and S.

UA 2403 RSW-ORD

I was #1 on the list. The app said F had checked-in full, but the website said 15/16.

The website was right, and I was upgraded while sitting in 7A.

And apologies to KER, J -- with whom I had a wonderful conversation while standing in the Group 1 line, and who was #2 on the list. She wanted my 7A if I got upgraded, and I told the gate agent, but I don't think she wanted to go through the effort.

I had to change the date and got booked into S as P wasn't available on my flight. SDC wasn't an option as I had to change 36 hours prior to my original flight. Got booked in Y. Was 1st on gate list and cleared at gate. Got refund in fare.

Well played, then! Many people make changes from P fares into Y during IRROPS and never get a dime back, nor a seat in the front.

3/4 now. Succeeded on JAX-IAD (2x pax) and MCO-IAD, and failed on BNA-IAD.

Likely to have IAD-TPA or IAD-JAX in December as my sole remaining CPU shot; UA has by and large conditioned me to buy F.

To put it in perspective, I was #28 on the list for ORD-DEN. :D

A few months back, Kacee had an excellent post suggesting that RM arbitrarily earmarks some flights as "ToD candidates" and aggressively refuses to clear instruments, while flights with similar loads and elite numbers on the same route clear substantially more upgrades.

I've observed this firsthand today, but with CPU rather than instrument-based upgrades. Flying a hub/hub route today, I had a rolling meeting that caused me to SDC back several times. Every flight I looked at was G9 and F>4 with essentially all F pax appearing on the upgrade list and very few uncleared pax: that is, quintessential signs of extremely low loads and even lower elite loads.

For no apparent reason, I cleared the CPU immediately on two flights I SDCd to, but despite spending some time on others with comparable (or even lower) loads -- one was 50% booked in J -- I didn't clear, and sat stubbornly on the list as a 1K on a W for several hours.

This is even weirder to me given reports in this thread that a new check-in on a flight should trigger a sweep.

So, either a series of strange coincidences, or RM does (somewhat arbitrarily) clear upgrades at a faster rate on some flights than others -- irrespective of load and other "typical" or predictable variables.