Deleting files doesn't reclaim disk space

(2.4) I deleted a whole bunch of (big) files but the space used on disk didn't change.

I've seem some very vague references to files being cleaned up only after a period of time has elapsed but nothing definite. Is this (a) true? (b) how long? and (c) can I override it to get back some space in a hurry?

EDIT:

Should add that these are automated backup files.