I know the wod rules since vampire the masquerade (and I know nwod and nwod 2.0). In all of the books I never saw any statement as to when players can advance that is stated out as a rule (I think some gm sections had rule of thumbs but not 100% on that but still as a rule it is never stated).
If that sounds confusing or unsatisfying you have to keep in mind that it is a storytelling system not a rpg. And even in RPGs there are not really rules for this (at least as far as I remember I never saw one stated there in any rpg I ever played). Per rules it is thus so as soon as you have enough experience points gathered and the gm allows it you can increase stats / buy new ones.
Although as it is a storytelling game there is one rule to keep in mind: That it should make sense. A bit of that is also indicated in an answer to your other question here: How many dots can be purchased at a time during advancement? I'll just use the excerpt there.
World of Darkness corebook, p. 217 (so far at this point of time there is no change in this in 2nd edition):
It takes time and study to learn new things. If a character does not already possess some basic competence in a Skill, he must spend time training in it before any dots can be purchased. The Storyteller decides just what he needs to do to qualify.
What that means is: If you are in an adventure where you have to run and climb alot it makes sense that during the adventure your character can increase his dexterity, strength or athletics skill BUT if it is an adventure that takes 1-3 ingame days to complete a change from athletics 1 to 5 is .... unrealistic without supernatural means and thus shouldn't be there (but its still up to the appropriate ST if he allows it or not as stated in that rules excerpt).
Or with other words: Character advancements can take place whenever the ST decides it is fitting into the story (the chars can for example train "off screen" during timeskips,....). Or supernatural powers can just "appear" as the character suddenly gains an idea / insight....but as stated before all of this is up to the ST as there are no hard rules saying:
- You can only advance during these times and not these
- You can only advance this far when spending XP