From what I've seen, even the base Green Guardian (or maybe the level 1 Green Guardian?) gives 1 extra turn, and it seems that all extra turns after that come in increments of 20 levels.
In any event, if you want to join a guild mainly for that purpose, it might be more helpful to look towards joining an already well advanced guild? Since you can only get so much task XP per week from what I understand about guild EXP, it would take a long time to build up enough for all the extra turns from a fresh guild.
Even a single turn can be extremely helpful, of course. Depending on luck and your skill with the minigame, you can get many extra turns in a row- I only have 9 turns at the start with my guild, and a REALLY lucky game was generous enough to let me get 2 vaults. Keep an eye for possible combos and you can drag out the worth of a single turn much, much farther than just one more move.