Essentially, I went about trying to figure out exactly how many years have passed between the Battle at Adachi Plain and the present day of Usagi. Here's what I did and the conclusion I arrived at:
One methodology I'm considering for determining this is to count the number of winters that pass across all the Usagi stories. I don't know that Stan himself would approve of this method, as winter tends to happen either to service the plot or to establish a tone, but it seems like an idea worth considering. The problem would remain, though, that we have no idea how much time transpired between Adachi Plain and the first Usagi story, and it's also risky to guess how much time passed across the Albedo and Critters stories before Usagi vol. 1 #1.
Okay, let's try this.
So, in Usagi's first appearance, he notes that The Battle of Adachi Plain was "so many years ago," so that obscures things a tad. But if we want to measure (approximately) how many years Usagi has been travelling since Albedo #2, here's my attempt to do so by chronicling the number of winters that are clearly visible in the Usagi stories:
Winter #1: "Usagi Yojimbo," Albedo #2 (November 1984)
Winter #2 "The Tower," Usagi Yojimbo v.1 #7 (March 1988)
Winter #3: "The Lizard's Tale," Usagi Yojimbo v.2 #6 (January 1994)
Note: "Lightening Strikes Twice," Usagi Yojimbo v.3 #6 (October 1996) includes a winter, but this occurs in flashback
Note: "The Withered Field," Usagi Yojimbo #7 (November 1996) features the onset of autumn
Winter #4: "A Promise in the Snow," Usagi Yojimbo v.3 #8 (December 1996)
"The Conspiracy of Eight," Usagi Yojimbo v.3 #9 (January 1997)
Note: "Three Seasons," Usagi Yojimbo v.3 #49 (June 2001) includes a winter, but this occurs in flashback
Note: "Vendetta," Usagi Yojimbo v.3 #53 (December 2001) is the onset of autumn and occurs almost exactly a year after "The Withered Field," Usagi Yojimbo #7 (November 1996)
Note: "Images from a Winter's Day," Usagi Yojimbo v.3 #55 (February 2002) indicates that it has been seven months since winter
Note: "Ice Runners," Usagi Yojimbo v.3 #142 (November 2011) takes place during the summer.
In summary, this approach was working for a while. A year in Usagi's world was taking 2-6 years in his, but there should be a winter somewhere around 2002, as this is six years after the last winter, as well as a year after autumn was clearly depicted in v.3 #53 and 54. There should probably be another one around 2008 so that it makes sense for it to be summer in November of 2011 (v.3 #142).
Granted, we've been shown numerous times (most clearly in "Three Seasons, v.3 #49) that Stan isn't showing us anywhere near all of Usagi's adventures. We skip over a lot. It's also possible that there were stories set during the Winter in 2002 and 2008, but that it was not snowing at the time. I would have to do a much more detailed search to examine the weather conditions of each Usagi story ever. I was just looking for snow
So, assuming there were two Winters we missed (a third should be coming soon -- remember we lost a few years to Stan's sabbatical after v.3 #144), there have been six winters (and, thus, six years) since the winter depicted in Albedo #2.
Update: As lobsterjohnson had indicated, Usagi #148 begins with a flashback to Autumn and ends in the present moment, which is Spring. This, of course, suggests that a winter has recently passed, confirming my estimation that one was just about due.
So really, the only leap of faith my timeline takes as of this point is that there was a winter we didn't see in 2002 and 2008.
Short answer: It would appear that six years have passed since Albedo #2. No way of determining how much time passed between Adachi Plain and Albedo #2.
