You've got to decide why you're doing the trip - what's your main goal? Are there any other things you want to achieve? It might sound daft, but, ask yourself too why are you going by bike.
For example, when I'm doing a bike trip on the bike, my only aim is to ride fun roads. So I steer clear of cities, big towns, and head for hills. I don't really care where I end up, as long as the tarmac is good. 200 miles is day is more than enough of that kind of riding. Others might want to go and see the Colisseum and visit the Uffizi and are happy to sit on motorways to do so; depending on your bike, 350 miles a day isn't that ambitious for that kind of riding. And then others are after a chilled time away from civilization; perhaps 50 or 100 miles a day is plenty for them. And then there are a few who want to travel as far as they can in the time available; who knows what they will cover in a day.
Anyway, there are some numbers: pick your profile and you'll have a guideline to help you start planning.