PLE pahh!
In the late 60’s it was a ‘Fool Line’ for the Lightning; IIRC about 350nm around Leuchars, with dotted lines for Lossie and other airfields (not all were approved for QRA ops except in an emergency). If you went outside the Fool Line you wouldn’t get back.
Time to intercept obviously depended on the distance flown, and of greater significance the distance of first ground / AEW contact. Thus with Norwegian alerting and a bit of guesswork on tracks, intercepts out to 300nm were possible. This was improved with the Tue/Thurs tanker training flights, which later became tanker Q flights.
There were also some routines, like 20 Jan every 4 years to the US East coast and back; also Bear flights to Cuba, and always a bevy of flights with a US fleet exchange in/out of the Med or a carrier fleet in the N Atlantic.
I recall one helpful civilian airline who reported an ‘air-miss’ (of several miles) on a Bear in the Faeroes Iceland gap.
Then there were a few ‘int’ alerted flights, where a telephone call would suggest a scramble in 2-3hrs (time for dinner in the mess); most of these were spot on.
On a few occasions there were ‘real’ scrambles, some with a buster call – supersonic. Generally these were ‘unidentified’ tracks (although probably expected) of aircraft returning from places North, and East of North; B 707, Victor, Canberra, and U2 (never confirmed). More often than not these resulted in a recall before intercept.