If the length of it is actually going to contain substance then I am all for it, but if I'm sat around for 1-2 minute long intermissions twice or three times each boss, in addition to having significant periods of time where I'm doing absolutely nothing but hitting what is effectively an oversized target dummy, waiting for the next mechanic to go off, then it's just generally all around a bad experience.
Ivalice and Void Ark series of raids had it nailed down perfectly.