It's a very personal, contextual and relative answer.

When I was a student it meant "how do I go on vacation and spend next to nothing?" For me that meant hitch-hiking, no accommodation and sleeping at the beach/in a park/camping wild, and spending the minimum for food. The hitch-hiking and not being prepared was the best part of that.

Later, when friends had moved abroad, it was short city trips visiting them, having a free bed, but buying a thank-you present; spending on drinks, food, entertainment and enjoying the local culture a bit more that way. Having a local guide that knew the city and local culture was the big bonus here.

Then I had a job with an international aspect to it and it meant taking advantage of the flight tickets paid by work once in a while and taking a few days extra to enjoy the local culture. Accommodation for the extra days was then paid by me. This wasn't always possible, but was nice to have it when it was.

With bouts of unemployment in between, it meant going camping and visiting friends abroad, cause it's cheap and all I needed was meeting my friends, or some nature and hiking in a forest, maybe some kayaking or renting a bike or scooter.

It all depends. I never had a big income or rich background. If I had, it would've been able to travel outside of Europe. To be honest, I could've afforded it with my current pay too, but I prefer to build on my savings and save for maybe one day buy my own house. I don't think you can define it as a percentage of your income. But maybe as a percentage of your entertainment budget? For some travelling is worth more than for others.