It's point-to-point flight time where the second point is not the first point. There are no distance requirements.
Ex 1. A to B distance 49 miles = .7 hours
Ex 2. Lengthy pattern work at A including trip to practice area 10 miles away = no credit
Ex 3. B to C distance 5 miles = .3 hours
Ex 4. A to A with no landings any other airports, distance 724 miles = no credit
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total Part 135 x-cty time: 1 hr
This is why CFIs always try and get at least one landing in at a second airport whenever they teach students. By doing this they get a massive amount of free Part 135 cross-country time even if the second airport is a mile away.