Unless they know that they are going to need the seats, like the Sunday after T-Day. Late afternoon, early evening flights with the preceding flights all full. AA offered $300 for SNA-SJC, same to all volunteers, about 1 hr before flight time. No flights available until Mon but I was looking forward to dinner with Mom more than a potential second bump. Already had the rental car reserved. US never seems to want to offer anything but free tickets and they make you wait until the flight leaves to get it. I always refused them. UA (actually UX and again on X-Mas day,) needed 26 volunteers when they downgraded equip from a 76 to 50 seater. $600 that time, though I did sit next to another pax who got only $400. FC seat on next flight, which also was downgtraded from a 320 to a 319, so technically, my assigned seat didn't exist, but there was another open in FC. I heard AA was offering $500 and a later same day flight in LHR for some flights on the Mon after T-Day.

I always find it best to get on the list early, even if it means going to another gate, before your gate opens, and having that GA put you on the list. They usually know which flights they are going to need volunteers on well in advance of the gate opening.

And always ask for your seat as reserved (or UPG'd) until they agree to compensation, that way you don't lose it if they end up not needing it. I did miss an UPG to FC from 10F on a 757, when they didn't need my seat. At least I didn't end up in the middle seat in the back.