10-15m from other locations
I didn't come across a specific distance when completing the wayfarer training recently. I hope this helps others with their nominations. As a pogo player I was unaware of existing ingress locations when nominating.
From niantic support
"In an effort to maintain reasonable Portal density, the Portal was removed by the automated system"...
..."The minimum distance between the 2 in-game locations should be at least 10-15mts."