Any time he wanted

"Aaaand...those are tits," Adrian sighed as he yanked up his top to examine the latest changes from the Role Exchanger that was rolling through his neighborhood.
Adrian had already got his mom's face with his younger sister's age. No telling who these breasts were from. At least he still had his dick.
His cock sprang to attention as he gazed down at the strange breasts. He grabbed one, exploring it by touch. It was supple and firm. Fun to play with.
He jiggled his tits a few times, turning his body back and forth so they bounced on his chest. His other hand began stroking his cock as he played with his new breasts, squeezing softly, enjoying the touch of his warm skin. His nipples spiked out in arousal and a pleasant, full-bodied warmth crept in, enhanced by the familiar need growing at the base of his cock.
He pinched his nipple, groping himself until he came, jetting white spurts of cum down his hand.
It had been a few minutes since the last exchange, which presumably meant that there would be no more. And that meant he would be stuck like this for the rest of his life.
Adrian supposed there were worse exchanges. A friend from school had received his dad's dick and his mom's sagging breasts. At least Adrian's youthful combination was much more pleasurable, and he still had his sexual orientation. Plus, now he could feel breasts any time he wanted. And nice ones, too. Maybe this wasn't so bad.

A freak accident swaps a young man into the body of his best friend’s girlfriend where he discovers his masochistic side in Better Than Ever, available on Smashwords or Amazon. Preview here.