As it was mentioned before it depends on your lifestyle and goals. My rides started out 10 - 15 miles then I got a new bike and decided to go on a tour
Now my closest average per ride would be about 19 miles, but upped it to about 30 when I was getting ready for the tour I was going on, then we threw in some 40-60 mile rides on a regular basis.
I'll plan on just going for an hour but usually end up going further once I am out
For some reason I can ride by myself for hours , the same old routes and not get bored. I am usually working on technique such as shifting/ spinning, standing to climb part of the hill, sprinting etc or just daydreaming and dawdling. I even notice the scenery once in awhile
It's about the journey and being in the moment, not about the destination