Hi!
I asked a similar question on this forum a few years back. As I model the late '40s-'50s, and am basically in a non-city situation, I have the HO scale poles about 16-18 inches apart. In the real world of that time, it may have been a little too far apart, for the typical poles were closer to 100 real feet apart.
But, I did some experimenting and found that the 16-18 inches looked appropriate for the layout, and so I stuck with that.
Modern railroads don't always have the "telephone poles", and from what I can tell they are further apart than what they used to be. Soooo, I come back to what I did - experiment with the placement and do what looks right to you.
Mobilman44