Like Chip said, separation depends on lots of variables...aircraft weight class, on an instrument approach, distance from the radar antenna, RVSM airspace/aircraft...
I'll give you the standard 1000' vertically or 3 nautical miles if you're less than 40 miles of the radar antenna or 5 NM outside of 40 miles.
Clear as mud right?!? I just go to work and give 'em 1000' and 3 or 5!
If you want to read more, read this
FAAO 7110.65 Section 5 Radar Separation.