[ptone]

Here is a quick script, if your backups are not stored in the standard location, or you want to keep more than 15 days worth of backups, change accordingly.

tell application "Finder"
	set OFBackup to ((path to (documents folder) as string) & "OmniFocus Backups:") as alias
	delete (every item of OFBackup whose creation date is less than ((current date) - (15 * days)))
end tell

Or for those that want to one line of shell script:

find ~/Documents/OmniFocus\ Backups -type d -name "*.ofocus" -mtime +15 -print0 | xargs -0 rm -r

You could add that one line to your /etc/daily.local file and it will run nightly.

(or create your own cron entry)

-P