Not very long

Erza began to use her tits to jerk Natsu off. He was honestly surprised with the level of energy. It matched what he expected from Lucy almost exactly.

"My eyes are up here!" She sassed, adopting a pouty face that felt very out of place for Erza.

It wasn't long before the door opened and Lucy entered, her clothing quickly being discarded.

"Alright, Natsu. I'm here" she huffed.

Erza turned to face her, and her expression dropped.

"BWAAAAH?!" She exclaimed "But... You're me! How is that possible?!"

Natsu couldn't help but stifle a giggle, as the real Lucy put her hands on her hips and glared at him.

"Natsu! What did you do?!" She snarled.

"Natsu! You did this?! Who IS she?! Why does she look like me?!" Erza added in the same tone.