We just got back two days ago from a week at Disney World and although I can't tell you about length of stay passes, I can tell you even at this slower time of year the
ECV's went quickly. For example, on Monday we entered the Magic Kingdom about 9:30 am and there were still some ECV's available. When we left at 3:00 there weren't any. I was talking to a guest using one of the Disney rentals while waiting for my husband, and she told me she'd had a hard time getting one arriving much past first thing in the morning.
I used an ECV the entire trip that I rented from an outside company. It was a great, great experience and hands down, I'd do it again. We stayed at Pop Century and I had plenty of room to park the
scooter in our room. We just moved the table a bit and one of the chairs and had the perfect space. I was so glad to have the ECV at the resort and downtown Disney as well as in the parks. I have bad knees and walking much makes them swell so with the ECV I had the chance to explore my own resort, many of the stores at Downtown Disney as well as some of the other resorts.
I should also report that getting around on the ECV was so easy. The bus drivers were all very nice and helpful. I did "crash" more than once backing into the buses but the drivers just laughed it off and helped me when I needed it. Most rides I was able to scooter right down to the entrance too and the cast members were very nice about it. I couldn't have taken the trip without the ECV and was so glad people were helpful and kind to me. I was very concerned that wouldn't be the case before I left home.
We arrived at about 3 pm on a Wednesday and my scooter was at my resort waiting for me. We left at 1 pm the following Tuesday and I turned my scooter back in just before leaving so I had it nearly every minute of my stay.
This was my experience and my suggestion, rent one or two scooters off site. You won't be disappointed.
Garland