The League itself is no longer canon, so you can't rely on lores like Jax', Nocturnes and Fiddlesticks' anymore, but I do think most of the releases around 2011-2014 followed a certain timeline. Well, earlier we had the Journal of Justice, which made it somehow clear, that the champions joined after another. The Kalamanda Incident is a good example for that. But now most of the releases tie in with other champions lores, and sometimes even change them (Azir: Sivir and Cassiopeia). Others change with reworks (Poppy is now much older than before her rework).
Some champions aren't tied into the current set of events, like Jhin. We don't know how long he is free and doing killings for the evil, mean, villainy part of the Ionian government. (But since there isn't everything released to his lore yet, we may find out in the future)
And finally events witch champion reveals: Aurelion Sodium
These are things that happen "right now" in lore.
That's as much as I know (and think is right) about this