CO compensation is, as noted, all over the map with one fairly constant metric of a max of $500 for a domestic flight. (I "scored" this amount for a EWR-PIT bump last Saturday.) Purely anecdotal, but I've seen higher comp amounts offered in EWR than in other markets.

I've been bumped as early as when the gate agent shows up (departure minus one hour) if the reaccomadation flight is leaving before the oversold flight in question. However, this is extremely rare.

They will solicit at kiosk check-in and/or when the gate agent signs in and sees the pax load for the flight.

I've also seen GAs show up at the gate knowing in advance that the flight is overbooked/oversold. A few times they have even identified, in advance, likely bump/re-route candidates.

Once I was on an oversold IAH-CLE flight and the agent was getting ready to process my VDB compensation. Another agent showed up and discovered that a pax was flying IAH-CLE-PIT. They ended up re-routing the PIT-bound pax direct (and gave no compensation). I got a "thank you for volunteering."