It's not how long she lasts that he's concerned about - it's how long he can keep her there, keep the instensity and the experience up that is his priority. He basks in the power flowing through the Force, sharing in it with her, while still using the Force to give them both buoyancy and avoid any accidental or near drowning that could ruin the whole thing.

However long she can stand it, he eventually floats back up to her, ending it by kissing her tenderly on the mouth while he holds her.