Ideal length of vacation is truly dictated by how long I'm able to take off of work. If I worked for a school district, like DW did, we would be very tempted to do a full month condo rental off site at WDW. Very tempted. My work is really the only thing stopping us. I always try to make sure our summer vacations are at least 9 or 10 days...start sometime on a Friday and go through Sunday of the following weekend. However, while I can't take a full month out of the office, my job does graciously allow me flexibility. I'm now able to take vacations that are 17 or 18 calendar days long, though I work some of it. For example, this upcoming trip...DW and the kids will pick me up from work next Thursday
around 1:30pm. We'll drive down and get to our condo by 9pm, hopefully. I have to work Friday until about 3:45pm, so we'll hit MK at night. Have Sat/Sun off, then work until 4:00pm on Monday...so again Monday will be parks at night. Tue is July 4th. Then I'm on true "vacation" from work Wed-Fri and the entire next week, we'll drive home that Sunday afternoon/evening, and I'll go back to work Monday. For Thanksgiving and Christmas/New Years, I'll do similar things...I have to work Thanksgiving week, but only Mon-Tue, so we'll leave Friday before and stay until Sunday after.
Of course, not all trips are like that. In our previous AP year, we did a few long weekends...arrived very late Thursday night, came home Monday.
Once this upcoming AP is over, we may look to do a beach theme vacation the following year. That'll be probably 8-9 days.