Zessee
Gonzo has it.
It very much depends on the wind, the aircraft types involved and, I have to say, the operators too. If the next landing aircraft is a B757 or BA146 (very slow inside 4DME) and all the operators are Heathrow based, and the wind is such that the landers are routinely making the optimum turn off, 4nm would be the absolute minimum that I would even consider; but it would very much depend on the day. At Gatwick, 2 B737s in a 6nm gap used to be routine. (I once managed 4 heavies in a 12nm gap).
The legal requirement is that the next landing aircraft doesn not cross the landing threshold unless the preceding departure is 2,000m down the runway if airborne or, 2,500m if it is still on the runway and rolling.
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