There's no stock answer. Much of it depends upon the airports you are traveling from/to. Also, your specific elite level. For the US3 being at the bottom tier basically means you will rarely ever see an upgrade. AA requires the 2 lower levels to use either earned or purchased stickers. While UA and DL do not that means lower elite levels will always request upgrades thus lowering the chances of getting upgrade.

Finally, in the US airlines are increasingly monetizing the domestic first cabin meaning less upgrades, even if you're at a higher elite level. I can very easily see free upgrades going away at some future points. Airlines are leaving too much money on the table.