I don’t mean the continual “touring” that has been groups do to pay the rent. I mean a big deal, stadium filling tour by a group at it’s height. Is this the longest tour ever?
Well, the young Mozart packed houses during his 3.5 year tour through Europe… and he was only 8-11 at the time. He then turned around and spent a year giving concerts in Vienna, followed by a stretch where he spent over 2 years in Italy (three trips - the “two years” is cumulative trip time). He stayed at home for a few years after that, then spent another two years travelling Germany and France (his mother, who accompanied him, died in Paris. He returned home about 6 months later).
Blue Oyster Cult has been on what they call the “On Tour Forever” tour since the mid-'90s.
According to some, Bob Dylan has been on a “Never Ending Tour” since mid-1988.
Metallica was pretty well known for releasing an album and then touring for years to support it.
Grateful Dead toured all the time with no real “new release” to pimp. More of a happening than concert. Ask anyone.
heh
Those were the days. 
I just saw the RHCP at Voodoo Festival in New Orleans, great show, and that was pretty much of a happening in its way, too. They have been touring a long time.