For me, I usually make the first day the longest day (700-750 miles) and then drop that down to around 400-500 miles after that. But I agree that it completely depends on where you are riding. Interstate is easier to get long milage days, twisty secondary roads you will be lucky to get 300 miles. As eye.surgeon said make it about time not miles, 8-10 hours will be plenty.
Hope this helps.
09 "Wee"-Strom DL650 (Orange)
99 Harley-Davidson XL1200S
73 Bultaco Sherpa-T
69 Triumph Bonnie (Project Bike)