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For years, they seemed to space their US Tours (at least) 3 years apart, giving them lots of new material for each tour, and letting anticipation build. I think one of the reasons their concerts were really 'events'.
1969, 1972, 1975, 1978, 1981
There were a few odd shows that didn't conform to that schedule, but for the most part 3 year breaks were the norm, at least in the US.