I would have thought three or four days without a 02 was quite rare.
During my time ('66 - '69) I don't recall ever having to roll the tyres on QRA, but a seven day limit would sound right. We hardly went more than three days without a 02. As to the combat spare, while Waddington was still converting from B1A to B2 there would be a mixture of B2s and B1As on the QRA dispersal. We never used it while I was on QRA but I would imagine that if a B2 went U/S you could almost guarantee the spare would be a B1A and vice-versa. SNAFU as they say.