Actually, the quote below is not true for the DropBox version 0.7.110 that most people use:
Your cache may contain directories older than 3 days.
only remote deletions are moved to the cache (and only stay for 3 days :-)
My cache was only half a gigabyte (about 10% of my actual dropbox size), but many people report much bigger caches.
It is easy to clear your DropBox cache, no matter on what OS you use it.