Not sure if you read my other post, I just talked a bit about this. This summer, we spent 7 days in Disney with 2 rest days (well, 3 if you include the last day we spent packing up the house, cleaning etc.... lol). But, we drove to Florida, took 3 days. So... in the end we were away from home 16 days, with 6 days traveling to and from and 10 days in Florida, 7 in parks. I admit, we were FRIED by the end. We had to cut our last day in parks (AK) short because we were shot, and I had considered leaving for home even before then. Not because we felt we'd had enough time in parks (definitely not), but because we were just tired, and we weren't smart about how we toured: we spent 36 hours travel time getting there - we arrived at 4:00 pm, then had to unload, pack, do groceries etc... were up until 1:00 am waiting for the in-laws to arrive who had flown, then jumped up and were out the door the next morning for 9:00 at Epcot. That was mistake #1. We should have spent all, or at least most of that day resting, and then maybe gone into a park for a few hours late that afternoon or evening. Then, we did four very days in the parks in a row, without a break. One of those days was a very full open to close (we did go back and rest for a couple hours) at HS, which by the time we got out of the park and home was 1:00 am. That was mistake #2. So, by the time we had our first rest day, we'd already been on the go between driving and Disney, for 7 days straight. Big, big mistake.
How long is too long, is a personal decision. Do you get homesick? That's a factor. But as far as worrying about tiring yourself out, I think the big thing is, if you're going to be spending a lot of time at Disney - tour smartly. Plan rest days. They're a necessity. But, the other beauty of being able to take longer, is I think it relieves some of the pressure of feeling like you have to cram your days full, in order to "see and do it all". Our next trip we'll hopefully have 12 days in Florida. We'll be planning smart - and only doing 2 days in a row without a rest day, so 8 park days, 4 rest days. But, the beauty of that is, is towards the end, if we find we're holding up better than expected, we can always use one of those rest days for another park day. Longer trips just give you more flexibility.