Final 747 design was done in 1965, first flight 1969. I think the A380's taking about the same amount of time.
Hardest part is defining the airplane - what do airlines want, what will they pay for, etc. That alone can take 10 years or more. Boeing knew for a long time that airlines wanted a bigger airplane than the 707 but they went through all sorts of designs until they settled on one that they thought would satisfy everybody (cargo, military and passenger airlines alike). That was 1965, Pan Am placed the first order in 1966, and in fact Boeing then waited until it had more orders before they even committed to building the airplane (they came pretty quickly though, only a couple months later).
First flight was in February 1969, certification came in December 1969 and revenue service started in January of 1970. So it happens quicker than it sounds like you think; about four-five years from the design of the airplane to revenue service. I don't think the A380's being rushed.
My source for this is the book Airliners, by William Green and Gordon Swanborough.
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