I wonder what kind of separation are we talking about? is it 2.5 miles? and are we talking about differing types? i.e. A320 on a shortish final..and a ATR about to depart etc.
Examples: ATR departing straight ahead, A320 on final, ATR must commence roll before the A320 crosses a 7 mile final. Becomes 5 miles if the ATR is on a side SID.
A320 (or Boeing) ahead of another jet on final, the distance is 4 miles. Slow a/c (eg C208) ahead of a jet on final, straight ahead SID, required distance is a whopping 12 miles. We are not permitted to shorten that distance.
The types that regularly operate IFR range from Caravan to A320/B737. On a day when the conditions are below circling for all, delays become substantial.
There's a tabular distance promulgated for all combinations of types/SIDS, and it's different for each runway.
The minimum radar separation is 3 miles.
The whole thing is a bit of a blunt tool, and there is, at times, a lot of wasted airspace. For example, no allowance is made for the fact that a lower performance departure might climb like a rocket. Nor for the fact that, although the speed of approach tested on the sim is 175, most a/c will be 10-30kt slower than that for the last 4 miles of the approach. Ironically, if a departure can accept a visual departure - which is surprisingly often even when below circling conditions exist for cat C - then visual separation may be applied up to the point geographical exists, or (as happens 99% of the time) the arriving a/c has landed.