I'm not familiar with mangas or anything, but I always thought the timeline was set by the timing of the game's releases.
RBY (also being the original set) (FRLG are here too)
GSC (three years later)
RSE (I always thought it would be about five to seven years later)
I'm not really familiar with the latter games, so I prefer not to mention them.
I base my thought on common knowledge on the games and the storylines, the technological and graphical developments felt from game to game, and also the amount of knowledge and experience shown from Professor Oak and the Pokédex enthusiasts throughout.
For instance, in the first series, there were 151 Pokémon known to the professor. According to FRLG, he only then (by the time Red becomes the Champ) discovers the existence of other Pokémon (the Johto gen). My thinking is he then explored Red and Blue's Pokédex, eventually met Prof. Elm in Johto, and three years later the GSC adventures were possible. An AIDE of his mentioned he had a radio show in Kanto in FRLG, that could have been the start of his radiophonic career, that eventually led him to the biggest radio station known in Pokéland, in Goldenrod. Now a lot of Kanto and Johto Pokémon are present in RSE, thus they are well know about, and thus their region seem knowledgable as well. Accordingly, these games are set a few years after the first adventures (and assuming that NO ONE knows more about Pokémon than Prof. Oak. NO ONE! Grr).
Did any of that make sense to you guys?