-
Depends on what time you want to start and stop (i.e. how many hours you want to bike) and how fit you are.
When I was touring through France, I hopped on my bike at 6 in the morning (because of the heat and because I liked it). That gave me loads of time to comfortably ride 100k per day, have plenty of breaks, and arrive at my destination around 3 in the afternoon (sometimes earlier). This way I had more than enough time to set up my tent, get groceries, etc without rushing.
After I found this out, I just planned my destination ahead of time, but also looked for possible destinations closer and farther (in case that was needed). I stayed at campsites though, which I never booked in advance. That gives you a lot of freedom when it comes to how far you 'need' to go each day (like @Heldring suggests).
Question for the hive mind on distances. I've not done any touring before and I'm looking at a multi-day ride along canals in France. Most of the itineraries I've seen are clearly aimed at bike hire / non-cyclists -i.e.: 21 km a day. They are great for planning stopping points but otherwise useless.
For a flat terrain and sedate pace what should I plan for each leg? ~50km a day? 70km? I don't want to undershoot and end up arriving at booked accommodation at 2pm everyday. Equally, I don't want to skip past stuff because I have too much distance to travel. Thoughts?