Looking back at major candidate campaigns since 2000, 70 days appears to be the shortest, by at least a week. Jim Gilmore's 2008 bid lasted 79 days. Tim Pawlenty hung on for 83 days in 2012.
We've excluded the winners from that graphic for the obvious reason that the winners didn't end their campaigns early. The longest non-winning campaign was the sorta-campaign of Mike Gravel in 2008. There's often no relation to the length of the bid and its success; in 2000, John McCain had the shortest non-winning campaign on the Republican side, but he still did fairly well.
In most cases, you'll notice that candidates didn't start dropping out in large numbers until the year of the election — in other words, after they'd lost a few actual contests. That's part of the reason why Walker's race is exceptional: He didn't even make it to the Iowa caucuses.
This is not set in stone. Walker could still be passed as having the shortest presidential campaign in recent history. All he needs is for John Kasich to drop out within the next six days.