In the rules, its stated that if the person who gets the next riddle doesn't post in 20 minutes, then the next riddle is up for grabs. However, Chargewithsword doesn't check this thread to enforce his own rules, so I'm the one who normally does. I'm a bit lenient on that rule, and I give them about a week to post another one...
Really, its up to you whether or not to post another riddle after those 20 minutes.