I don't think the length of the trip matters. It is the frame of mind you have when you leave the dock. My first cruise in 97 was only 4 days and as my wife had to drag me on the ship I did not enjoy it. I had always felt like Jimmy Buffet, "I'll never see the islands from a tacky cruise ship."
In 2005 she convinced me to go again and I left open minded and had a great time and am now hooked. Our cruise in March is 8 days and I look forward to being able to do a back to back.
Whether 5 or 7 days, go looking forward to it and you will have a great time