According with the game, Alduin is reviving dragons buried in the dragon mounds. So, how long it takes until all of them are alive?
Are you asking about dragons in general? – then Mge Xy’s answer should do. But in case you are asking about their buried types (named ones), that Alduin is reviving with a shout, I think that if you follow the order on the map that is given to you by Farengar Secret-Fire you will get all the buried dragons revived. The first one was near Windhelm, between miners settlement and orsimer’s stronghold. That's all I remember unfortunately for I’ve lost this map the next day after I received it (put it on a display in my Hearthfire DLC house and it fall through the textures while loading the next time I came).
In case You meant the exact amount of time between dragon attacks - my 105 lvl character (adept lvl difficulty) was always (through all the 105 lvls and 10k+ hrs of gameplay) attacked after 3-5 fast travels/visited locations as I was passing by random NPC spawn areas (dragons, bandits, paddlers, daedra, vampires and etc.). So I literally knew beforehand that this time it is ok not to save game and that time – I’d better save it in case dragon won’t pick up the fight (those cowards x_X).
As noted, there are an infinite number of generic dragons. Once you flip the trigger to start their spawning, they'll never stop unless you use a cheat/mod/et cetera to do so. So in that respect, they will never "all" be resurrected.
That said, there are a fixed number of named dragons in the base game. Four of these are known be alive at the start of the story (one of whom does not appear in Skyrim). Five more have known burial sites and are brought back by Alduin during the course of the main quest. They are:
- Sahloknir at Kynesgrove, during A Blade in the Dark.
- Nahagliiv at Rorikstead, during A Cornered Rat.
- Odahviing near Riften, during The Fallen.
- Viinturuth, near Lake Yorgrim, after The Throat of the World.
- Vuljotnaak, near Sunderstone Gorge, during Diplomatic Immunity.
Of these, Odahviing is the last you'll encounter during the course of the main quest. Thus, it takes until the event of The Fallen before all the dragons Alduin will wake up are, in fact, awake.
The remaining base-game dragon, Vulthuryol can be found in Blackreach and presumably has been alive since before the events of story.