Watched Land of the Dead last night and was shocked to see that no, Land actually takes place 3
months into the ZA ... ... ... ... oh, wait, no it doesn't. :D
"Three years" is specifically referenced twice in the film - once by the garage owner, and once by Cholo (who has been working for Kaufman for three years). So, arguably, three years is the
minium amount of time passed. It could be a little longer, but considering that you have two clear references to "three years", then that's what Romero's telling the audience quite clearly.
Also - good video pull - Romero had always talked about Land being the living 'coming back' from the brink as well as the evolution of the zombies' abilities and awareness (building upon what began in Day). Bub, from Day, would be a considerable
regression from Big Daddy & Co ... and considering that Bub's story in Day isn't about a smart zombie becoming dumb, but a dumb zombie becoming smart, well, there you go - the narrative direction is clear as an azure sky...
https://youtu.be/OsrvURzZmDM?t=185