I usually try to book for at least 14 days or more and that usually means two back to back 7 day cruises. That way I can cut down slightly the effect of the single supplement by more cost-effective flying.
I think I went overboard this fall, since I'm away from home from Oct 13 to Dec 3. Just the logistics of making sure I have pre-paid all bills likely to come during that time is getting a bit tedious. It also means I can't use a credit card during the first few weeks because that bill would arrive before I get back. I will be carrying a lot of travellers checks I guess. I had forgotten how tedious this can be. Although I've done a couple of seven weeks trips in the past, it has been a decade since I last did it.
I've also booked back to back on Victory next february! Have not booked flights yet though, so I can add an extra cruise on the end if I find a good one. The single supplement stops me from adding a Princess cruise - I simply refuse to pay double rate.