PHB says 'once per turn,' so, it would seem that a Rogue with Sentinel, for example, could "consistently" deal Sneak attack twice per round (whether the enemy triggers an OA or the melee weapon attack ...
In 3.5e, I used to add Sneak Attack damages to all the attacks of my character as long as the target was flat-footed or flanked or... However, when discussing the feature with wax eagle he mentioned ...
In the Pathfinder rules here's what it says: If a rogue can catch an opponent when he is unable to defend himself effectively from her attack, she can strike a vital spot for extra damage. ...