It's hard to talk about patterns in the time between tours, because there are many factors involved. For instance it was just under 3 years and 4 months between the end of the Elevation tour on December 2, 2001 in Miami and the beginning of the Vertigo tour on March 26, 2005 in Los Angeles. This was not the original plan though.
What became the Vertigo tour was originally intended to start a full year earlier, in March to April of 2004 when the album was originally planned for an early 2004 release. I was told this personally by Bono's bodyguard John Sampson in November 2002 when he was working on the Rolling Stones tour.
As far back as the fall of 2002, they had the 2004 tour pencilled in. When they decided in late 2003 that the album was not what they wanted, everything got pushed back with the album arriving about 8 months late and the tour 1 year late.
Also U2 was originally going to do a 6 week tour of stadiums in Europe in the summer of 2002, so had they done this, the gap between Elevation and Vertigo could have been as little as 1.5 years from Sept 2002 to March 2004, or 2.5 years with Vertigo being pushed as it was back to 2005.
The biggest pattern is that with most tours, U2 have strong intentions that either get pushed back or don't materialize at all. Exampples follow:
Vertigo: pushed back a full year from the original starting date. Also, they looked into stadium shows in North America, and possibly Europe as well, for summer 2006, which are no longer happening.
Elevation: originally intended dates in Australia and Japan in Nov-Dec 2001 and South Ameirca in January 2002 were dropped and summer 2002 European tour plans cancelled by shortly after the Grammy Awards in Feb 2002. Also this entire tour could have happened 1 year earlier if Leave Behind's originally planned early 2000 release had occured, so Elevation was another tour pushed back an entire year from originally intended.
Popmart: was originally talked about in fall of 1995, when Passengers was released, as starting in fall 1996. It of course was moved back to April 1997 when it became obvious Pop could not be released until 1997.
Zoo TV: the November-December 1993 shows in Australia, new Zealand and Japan were originally intended for the same time frame in 1992, but were pushed back an entire year to become the ending of the Zoo TV tour when they were originally only going to be the final shows of 1992, while European stadiums in summer 1993 was always the intention for Zoo TV from the start.
Love Comes To Town: this tour was mainly shows in Australia, New Zealand and Japan, which did not start until September 1989. It was originally intended to be the 4th leg of the Joshua Tree tour and was to have occured at the beginning of 1988. They decided they were getting tired of being on the road and also had started work of recording new songs for Rattle and Hum, so the idea then became to have those countries get their own tour for Rattle and Hum which was to have started around March 1989, but then that was pushed back 6 more months.
As far as the next tour goes, there have been some rumors of a new u2 album by the end of this year. I really doubt it will happen, because I don't think U2 will release a full blown album without following it up with a tour. Since 1987 all of their tours have started in the spring. I don't picture U2 wanting to go back on the road only 1 year after Vertigo ends, unless they intentionally want to do a much lower key album and shorter tour, perhaps only in stadiums. I would expect a new album in fall 2007 or spring 2008 with a tour to start in the spring of 2008.
For Atomic Bomb I think U2 was in no hurry to record it, because they knew they were not going to go on the road til spring 2004, so back in 2002, they felt no rush to record. They assumed all the work they did in 2003 would be enough to have a finished album delivered to the label by Christmas 2003 for early 2004 release, but they ran out of time by using this slow approach, causing everything to get pushed back.
