I would think a lot depends on the type of bike and the the type of roads. Since I ride a mountain bike on hilly gravel roads, my long rides are currently in the 25-50 mile range (last year was the 16-25 range) I hope by the end of the summer, my long rides will be in the 50-75 mile range.
If I were riding a road bike in a less hilly place, I would probably double those numbers.