Is there a rule-of-thumb for estimating distance to an aeroplane?
A rule-of-thumb would imply frequent usage. Such as this list on SKYbrary.aero: Rules of Thumb.

For usage by pilots and/or controllers, a distance-to won't be beneficial even if there was a rule-of-thumb (there isn't).


Imagine two planes on approach to a runway, the controller uses his thumb, now the following plane is also higher, so we have different slant angles: it just won't work to judge their separation (and on a foggy day it's of no use of course).
Imagine a pilot flying VFR, at night. The pilot spots the navigation lights of a plane, but nothing else to judge the plane's size: again won't work.