Indeed - the answer is almost infinite. In N scale it would be anywhere from 6" to over 100 feet radius! (However usually matching the diverging closure rail radius in the reverse mirror image).
At issue is where do you start the curve (how far from the frog point); AND how far away till you reach the parallel point; AND what is the center line separation. Of course too gradual will mean really wasted space. To sharp will make for reliability and operational problems.
-ed-
_________________
-ed mccamey-
COSLAR RR -
http://www.coslar.us/NMRA Standards and Conformance Department
PROTO & FINE Scale Coordinator
I estimate I have about 5 pounds of coupler springs somewhere in the vicinity of my workbench.