Celebrim
Legend
As far as I know trebuchets were not really used to knock down walls, but to shoot over them to damage whatever they protected. They still had a rather low velocity and damage to the wall was rather low so it took a long time to destroy them.
While it certainly took a trebuchet a long time to reduce a wall, it took less time to reduce a wall with a steady barrage of stones than it took to starve out a castle and in the long run success was basically inevitable unlike other risky approaches like sapping. Trebuchet were used on many occasions to reduce the walls of a city or castle between their introduction in the 11th century and the ascendency of cannon in the 15th century. The most famous example I can think of is Edward the Longshanks used a large Trebuchet to destroy the gatehouse of Sterling Castle in 1304, an incident that basically marked the end the high walled castle as an effective deterrent as it proved that with a big enough engine, it was only a matter of time before you could reduce basically any castle.
The Mongols also used Trebuchet to knock down the walls of Bagdad.
I thought I had mentioned star forts at some point, but maybe that was a different thread.