Right away. It's not that the icon irritates me (I have it hidden anyway), it's more so a fear that the files are still taking up space, even though I know they have been compressed. I just want them gone, off my computer for good.
On my mac, every two months or so. Macs get so slow with a lot of files in there. Last month I had about 220,000 files in there
I wrote a windows file system watcher service to clean my bin in case I forget to shift-del but it does a few other things with temp folders and log files and real time backup. It looks sort of like the windows firewall service but you set rules for files and directories. Instead of just manually doing it I ended up writing this service that I keep adding stuff to from time to time.