Not too long, but that's OK...

She kind of laughs a little as she takes in the moment.
"That was as easy as the anal cream pie you like us to make in your ass," Amy tells you. "You will like it best when we whip it into a froth by taking turns. Let's do that next, girls. Fill that cavernous butthole with enough foamy cum to make it boil out and over his cheeks. We can lean him upside down and let it run between his legs, over his big pregnant belly, between his big milky jugs and finally into his mouth, to taste our combined flavors. I'll even take a gallon container and fill it with his breastmilk and dump a few of them into the mix and maybe even a little of his own cum too."
You feel her swell up in your throat and pump load after load of cum directly into your stomach. After she finishes, she pulls out. She gets caught a few times due to her girth.
"Don't worry baby, these can go the extra mile--", she interrupts herself, "the extra gallon for you! Plenty of cum left to cream pie your butt like I said!"
"Uh...occupado?" You point to Red.
"Sorry Master, I'll come out now if you would lift up," Red mentions. You do and she pulls herself out.
Amy directs them to lean you back, ass in the air and legs spread.
"OK so all we have to do is first fill that thing with cum. You two start while I get something for his milk." Amy jets out, hard dick bouncing proudly, flinging drips of precum about. She quickly returns with an empty gallon jug with a picture of your face and generous upper body on it.
"Prepared as always?" You ask.
"I was going to be saving your milk anyways," she admits as she sticks one of your fat nipples into the top. "Milk yourself until it's full, sweetie," she giggles as she whacks her erection and swaggers over to stand above your asshole. Blue and Red had already cum in you once while Amy was gone. That allows them to step back as Amy shoots ropes at your gaping hole. She gets most of them in, too!
"Keep it up! It's big in there," she reminds them as you hear the wet sounds of her rubbing herself furiously again. Before five minutes is up, Amy gushes at you, both figuratively and literally.
"I am your biggest fan, in every way," she tells you as her cum floods into you for a second time. How much will even be needed to fill you?
"First milk is full," you say as you hand it off to Red who pours it into your butt, then replaces it at your other nipple. "I'm glad I can help," you tell them. Red and Blue dump another load into you, followed shortly by Amy's third and a second gallon of your breast milk.
"This is going to be great," Amy says as she manages her fourth load into you in under twenty minutes.
"I'm going to end up heavier than when Amy was in me," you observe as the weight of the fluids in your ass bears down on you.
"Of course, Master. A full ass of liquid will fill every gap, while Amy had a lot of empty space around her body. You will be fine in this position, Master."
Amy shoots a fifth into you.
"Wow Amy, horny much?!?" You ask incredulously.
"Oh yes, this boner is not going anywhere. These mammoth balls can churn out enough cum for this job, just not all in one orgasm. Here let me empty your milk."
"I sure can make a lot of it," you say as she replaces the container again. "I can see why you cum so much for me. This rack sure doesn't quit!"
"Yeah, ugh!" Amy cums again for you. "Whew, this is exhausting! When are you going to be full?"
"I'm betting it will take a long time. You had your entire body in me! Those balls are big, but if you want it done in only a few shots, they would need to be several times larger! You better pace yourself, Amy!"
"Arg why did I have to stretch you so much! This is gonna take forever! But I'm not giving up! Any suggestions, everyone?"
"I think he's right. It will take a while," Red says.
"I better pace myself, then..."
"Damn, I'm gonna be heavy after this..." you tell them, feeling how heavy you are now.