Well so much depends on how firm the ground, both for gaining roll on tee shots and for being able to occasionally bounce a long approach shot onto the green. My driver shots travel about 200 yard total if I make poor contact and about 215-220 yards when I hit it solidly, along with the once-a-month miracle shot that somehow finishes 250 yards from the tee. But say 210 on average.
I have never really enjoyed a round at anything beyond 6,700 yards (Par 72). On a links or heathland course in the UK that length would be no problem but on a typical USA parkland layout it's too long. I recently played a course that was in that neighborhood (albeit on a day when a handful of tees were moved up one box to let the normal teeing ground heal) and it was great fun but I was overmatched.
On my home course which tends toward the too-wet I find the normal "white" 6,200 yard tees absolutely correct for my game. I do not particularly enjoy moving up to the 5,800 yard tees and the few time I've played back at the 6,500 yard tees it's a grind.
So I conclude that absent firm and fast conditions 6,200 is optimum and the limit is around 6,500. Probably add a couple hundred yards on a firm linksy turf or subtract a couple hundred when the fairways are plugging.
Even so, there didn't seem to be the mix of lengths which I find so much fun on the most older English courses.
By the way, this last bit is very important. For me a 6,700 yard course that's a steady diet of 370-410 yard Par 4's is a slog and unless the Par 3's and Par 5's are truly awesome it will be a disappointing experience. OTOH, a 6,700 yard course can offer up some 340-350 yard holes offset by a couple of 420-430 yard monsters and I'll probably enjoy it. I just play the couple of long holes as really, really short Par 5's (for my game) and have at least one or two driver-iron holes per side enjoy and try to score on.