How electric poles are distributed and placed
Just to be clear, we're talking about medium and low-voltage distribution poles that are shared with other utilities, not high-voltage pylons. 

If there are no property constraints, the utility will space the poles as wide as cable droop (sometimes called 'catenary', for the shape of the droop) would allow: up to 300 feet in rural areas.

Generally though they will place poles on property boundaries, in coordination with local planning / zoning authorities. They will place them frequently enough so that down-feed connections to nearby homes are realistic. This works out to about 125 feet typically.

Related question here: Low voltage power lines - formula for spacing between poles?