~700 miles is the limit of comfort for me,.
Scotland trips normally start with a 550 mile first day - getting through Glen Coe before it's dark (to enjoy the view) is my usual goal - that's not uncomfortable. A Wales twip is normally 2-300 miles, barely noticeable.
On my last couple of trips to Norway the last day has been a long one too, Hirsthalls - Hook of Holland was ~650 miles in a day, not a great ride but not uncomfortable. The year before was ~1000 miles down sweden / oresund bridge / ferry to puttgarden / across germany & the netherlands. Finding myself on a fast five lane motorway in heavy morning commuter traffic after nearly 24 hours riding was absolutely horrible.
Normally on a trip I don't plan ahead where i'll be camping - so I limit my ride to a maximum of 10 hours a day, regardless of mileage. If you're not working to a fixed schedule I heartily recommend budgeting on hours, not miles.