There is a legal answer (in EASA land there is apparently no minimum distance) and an intent answer. I'll speak to intent.
Why do you think we are required, during training for a license or rating, to complete a cross-country exercise?
Is it to fly to a bunch of places we can see to rack up the hours or is to learn valuable skills about in-flight navigation, airfield procedures, and aircraft performance (to name a few)?
You could fly from your airport to a point x number of miles away then back to your airport, but what have you learned? You've learned how to depart and arrive at your local field and how to turn around.
Hour building for a license is not supposed to be just adding digits in a book for someone to look over during a flight test. Its meant to give you the skills and experience to demonstrate that you have the requisite knowledge to hold the license in question.
When I was instructing, I always relished the "I got lost" conversation when my student(s) got back from a cross-country, because we could then talk about how they got "unlost" and build up their confidence. It might have cost the student an extra couple of bucks, but what they learned from the experience was priceless.
So my advise to you is go to an airport far enough away that you're pushing your boundaries, get "lost," learn some great skills.