G'day, I have a question for all those in the know.

I am currently reading a story where a character has their wings clipped in punishment for breaking the law. I need to know, how devastating of a punishment is this? Are we looking at a few weeks to months of grounding before feathers grow back, or is this more along the lines of never flying again?

Nothing has, nor would be hinted to this in the show, so it depends on what it is the author wants to do. But typically limbs never grow back for mammals.

Some members of my extended family keep or have kept birds such as chickens, ducks, or pigeons.
When one is dealing with ordinary barnyard birds, some of the long primary wing feathers may be clipped to prevent the bird's being able to fly well. It might still be able to escape an intruding fox or other predator, but it won't be able to fly far enough in a panic to leave the farm and become lost. The feathers will grow back, in the normal process of moulting.

If the owner wished the effect to be permanent, the entire last joint of the wing can be removed. I understand this nasty bit of work is called "pinioning." I would suspect many a pegasus would prefer death.
Pinioning is sometimes used to keep decorative swans and geese in the ponds where they were placed, to swim about and look pretty.

Birds generally moult about once a year, but it varies by species. Some moult 2-3 times a year. So your timeframe can be pretty flexible, and the length of the punishment would be pretty variable, going from the time they're clipped with the clipped feathers not starting to grow back until the old ones have fallen out in the pony's next normal moulting. So, decide how often you want pegasi in your story to moult, and go from there.

"Time is bunk. Lunchtime doubly so."