The cost reduction applies only to generic mana in the cost of spells you cast of the chosen card type.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of Stenn's second ability). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

If Stenn's controller activates its last ability during a player's end step, the delayed triggered ability that returns it to the battlefield won't trigger until the next time an end step begins, usually during the next player's turn.