I don't know how it works in FRA, but I'll tell you the remote station procedure that might serve as a benchmark. LH usually looks for VDBs at check-in. They have an agent approach everybody in the economy class line detailing the offer that LH makes (the standard EU regulated minimum).
If you're booked in C or have status no agent will knowingly contact you about denied boarding, so you'll have to take the initiative. VDBs are processed by sequence number, and since there are usually many volunteers you need to get to the airport quite early to make the top of the list and actually have a chance to be denied boarding.
Gate agents decide whether they need the volunteers once they have the full list of checked-in passengers. They usually start by making upgrades quite early, moving pax into higher classes (C to F in needed to make room in biz, but mostly Y to C) as early as two hours before the flight, if they know there is a significant overbooking and a lot of C seats. If overbooking isn't significant, agents tend to wait: they wait to see how many no-shows they have, and most importantly they wait to see how many passengers will spend their miles or vouchers for upgrades. If upgrades don't free enough seats in Y, they will deny boarding according to sequence number of the VDBs.

In FRA and MUC I've been a volunteer twice, but that all happened at the gate: agents there wait until they get an idea of how many - if any - passengers from connecting flights will be late. If everything is more or less on time, they start asking for volunteers. I don't think a passenger would be approached at check-in in FRA or MUC. I've been told, but I'm not sure, that SEN are upgraded by default when a flight is overbooked, without the wait to see if there is actually a shortage of seats.

In 2004, about 5.5 million booked LH passengers didn't turn up for their flights (both no-shows and missed connections); approx. 650,000 passengers got a seat by over-booking. Each denied boarding was matched by about 13 additionally transported passengers. (check this out: http://konzern.lufthansa.com/print/v...=214&nlang=en).