It is not possible to decide this in the abstract. It all depends on how far it is to the next village and what amenities are there. For us, a short day would be 15 km, a long day 35 km or more.
Most of our very long days have been forced upon us by wrong information about the non-existent facilities in the villages ahead. There is a double pain in this – not only does it mean more hours of walking, but those extra hours are in the hottest part of the day.
On our fifteen trips to France we have walked 10,100 kms in 442 walking days; around 23 km per day, walking for an average of five hours each day.
Our speed whilst walking is about 5.5 km/hour, but with our many stops for photography, conversation, drinks of water and consultation of the map, our average speed is around 4.7 km/hour.
We also walk a surprising distance without packs after arriving at our destination – exploring the village, visiting the local restaurant, even going to and from the sanitaires. On average this amounts to about four kilometres per day.