Once you collect all the Dragonpriest mask and you get receive the final mask Konahrik I probably spelled that wrong. Anyway, once you receive that mask do you automatically obtain the conjure Dragonpriest spell?

How does that work, does the mask haft to be equipped to summon one? (I doubt that)

And how strong is the Dragonpriest you conjure, it is a exper spell like Dremora lord, and they do a lot of damage even on Master at lvl 56 (or enough damage for what it is anyway).

Edit: I did had totally forgot you could conjure a Dragonpriest, somehow that just slipped my mind but that really gives any caster based class plenty of a reason to get those damn mask.

Also what is the base duration of the spell without dual wield conjuration?

The ability to conjure a Dragon Priest is a random effect that is out of the control of the player. And yes, the mask needs to be equipped for the abilities to take effect.