It's not set down as a longitudinal (or anything else) separation when using radar - aircraft are just kept apart by a minimum distance. The standard is 5 NM but in areas of poor radar cover or if the aircraft is a long way from the radar head the distance might be increased, sometimes the increase might be in the lateral plane only because the positional accuracy of the return is reduced in such circumstances - for the old timers amongst us this is saying much the same as 'the blips get huge!'