My 1st cruise was 4 days "to see if I got sea sick". I didn't, and I remember saying, "if we ever do this again, it HAS to more than 4 days!!
Some 16 cruises later, I am up to 10 and 13 day cruises, but 7 seems to be the norm. If you're a working person, it's hard to get more than a week off at a time, and if you're flying in from somewhere far like we are, anything less than 7 days makes no sense.
My suggestion is go for the 7 day to start, and ENJOY!!