Short answer: 15 to 18 ft between poles. Closer than that and the lights hang almost flat, which looks like a washing line. Further apart and the middle of the run sags low enough to walk into.
Why that range and not more
A string of lights hangs in a curve, and the deeper the curve the lower the middle sits. Over a 15 to 18 ft span with a 10 ft pole at each end, the lowest point of the droop still clears head height comfortably. Stretch the same run to 25 ft and one of two things happens: either you pull the cable tight to stop the sag, which puts strain on the poles and kills the look, or you let it hang and the middle drops to around shoulder height.
How to measure before you buy
- Walk the edge of the area you want lit and mark where you want each pole.
- Measure the gaps. Any gap over 18 ft needs a pole in the middle.
- Count the runs, then add one. Two poles make one run, three make two, four make three. Neighboring runs share a pole.
Adjusting the droop
If the sag is deeper than you want, the usual fix is not to move the poles. Shorten the length of string between them, or take a loop out at the hook. A little droop is what makes string lights read as string lights rather than as a cable.
Crossing runs
If you are lighting a square patio rather than an edge, run lines across the middle as well. Four poles at the corners plus crossing runs gives you a canopy instead of a border, and the light lands on the table instead of around it.