How many times per day can a coven use the gained spell=like abilities? There doesn't seem to be any wording that limits it, but, for example, Witchfire covens add blight 3/day to the list that already contains a (seemingly) unlimited blight at the same DC.
When three hags of any type gather, they can form a coven to gain increased magical ability. Any combination of hags can form a coven, but green hags are the most common members of such foul gatherings.Whenever all three hags of a particular coven are within 10 feet of one another, all three of them can work together to use any of the following spell-like abilities: animate dead, baleful polymorph (DC 18), blight (DC 17), bestow curse (DC 17), clairaudience/clairvoyance, charm monster (DC 17), commune, control weather, dream, forcecage, mind blank, mirage arcana (DC 18), reincarnate, speak with dead, veil (DC 19), vision.
All three hags must take a full-round action to take part in this form of cooperative magic. All coven spell-like abilities function at CL 9th (or at the highest CL available to the most powerful hag in the coven). The save DCs are Charisma-based, and function as if with a Charisma score of 16 unless one of the hags has a higher Charisma score, in which case the spell-like ability DCs are adjusted by that hag's Charisma modifier.
Witchfires occasionally join or subjugate existing hag covens (see page 167 of the Pathfinder RPG Bestiary). A hag coven that includes a witchfire gains the following additional coven spell-like abilities: 3/day—blight, create undead, fire storm (DC 21), nightmare (DC 18), waves of exhaustion (DC 20). All abilities function at CL 9th, and save DCs are based on a Charisma score of 16. The use of these abilities functions identically to other coven abilities. Such covens must have at least one living hag, as covens of three witchfires gain no coven-related abilities.
On a semi-related note, has it ever been established anywhere that the witchfire's witchflame bolt is supposed to be ranged touch or not?