For the UK, have a look here:
http://www.caa.co.uk/docs/33/CAP493Part1cor.pdf
CAP 493 - Manual Of Air Traffic Services (MATS), Part 1.
Section 1 (Air Traffic Services)
Chapter 3 (Separation Standards)
It doesn't give much away as it would ultimately be laid down in the controlling unit's MATS Part 2, or in the case of the Military. Military Manual of Air Traffic Management (MMATM).
AFAIR its 5nm in most places and some areas its increased to 10nm because of radar coverage.
I hope that helps.
MM