“C-Can I let it out now, Princess?”

Celestia watches the strain pulling on Twilight’s expression for a moment, holding her hoof against the tightness of her full belly.

She can almost feel those three hayburger combo meals churning around inside. She was impressed her student finished off all three so well. Truly a pony after her own heart.

“Very well. Release.”

Twilight takes her cue and relaxes the tension on her plothole. Without even a second of hesitation a loud and abrasive wind forces out in a thick steaming cloud from between her cheeks.

It continues. 
A whole lot of fast food and a little touch of inflation magic from Celestia and Twilight feels she could fart for an eternity. 

Her head sinks into the mound of pillows below her, loosing a dampened groan through them. 

The gas spews out of her like an emptying pressure cooker; a rich fog smelling of mustard-oiled onions. Fatty and sharp. Hayburgers left out to spoil in the sun.

Celestia leans forward just an inch to feel the hot rush of air in a geyser against her face. She breathes it in, nose scrunched and eyes squinted.

“Somepony needs to improve their diet, I think. Not nearly enough greens.” 

With a giggle she wiggles the purple ass cheek closest to her and almost interrupts the stream of gas for a moment.

Twilight moans again, half filled with despair and half satisfaction. She’s already at six seconds and even if her flatus is quiet, it’s showing no signs of stopping.

“Come on. Keep it going. Just a little longer.” The princess encourages, and Twilight tries her hardest.

With every impulse for her sphincter to tighten, she keeps it slack. 
It now resembles a loose exhaust nozzle, heaping on to the cloud of fetid fumes hanging like a curtain over the atmosphere in the room.

“Longer. You can do this, dearest. Just keep it going; don’t stop.” 

Celestia’s voice is like the comforting call of a siren, but Twilight’s will loses to her body.

At just under twelve seconds, it rises into a squeal and cuts off abruptly.

Twilight is a heaving, panting mess of exhaustion.

“I’m... s-sorry... Celestia... I... tried.”

“Oh, don’t be sorry. That was quite impressive. I fear my Orchids will bear the worst of it.” Her eyes fix worryingly to the flowers in the corner of the room, half expecting to see them wilted.

Twilight turns her face from the pillow to smile wearily at her mentor.

“Now you’ve finished, it only seems right I let loose a little myself.”

The pocket of gas has been steadily snowballing in her lower gut, and with a graceful sigh Celestia allows it to pass. 

 It starts quietly; so quietly Twilight isn’t even sure there is gas escaping, but within only a few seconds it builds into a gentle wet patter; like the purr of an idling engine.

Twilight watches Celestia’s tail flutter. This is the only telling sign she is even passing wind at all, though; her face is relaxed into soft effortless contentment. For her, this is as easy as breathing in and out.

Twilight watches both impressed and a little scared as it grows louder and more thunderous for a further fifteen whole seconds. 
She follows her unfailing internal clock as she listens, and decides that when it finally rumbles to a gratifying end, it has reached twenty one seconds.

The smell of Twilight’s gas has been eclipsed by Celestia’s; a rich tapestry colored with tones of rancid veggies, curdled dairy and hot, brow-singeing sulfur. Souvenirs from Celestia’s perfectly-balanced diet.

“Goodness! I DO apologize!”

Twilight wafts her hoof in front of her face as respectfully as she can. “Don’t worry! It’s not too smel-“

“That was a pathetic display. Not even thirty seconds?” She scolds herself and clicks her tongue.

“Let me set a better example...”