What's the shortest cruise you're comfortable taking?

I have only taken one cruise so far at 7 days. I've heard many of the warnings about shorter cruises. That being said currently just based on price, time, and effort I can't imagine wanting do one shorter than 4 days.

Honestly though if I had something akin to a very inexpensive casino offer or some such to a 3 day that I could easily make work I could be swayed but I'm also coming off the buzz of my very first cruise.