Is homework that requires at least 12" length of parchment anything to complain about?

A 12 inch essay is on its own not something to complain about. Consider that in the past they had been assigned an essay three times as long, and Hermione has written an essay four and a half times as long in tiny writing. From Chapter Nine of Chamber of Secrets:

Harry found Ron at the back of the library, measuring his History of Magic homework. Professor Binns had asked for a three-foot-long composition on “The Medieval Assembly of European Wizards.”

“I don’t believe it, I’m still eight inches short....” said Ron furiously, letting go of his parchment, which sprang back into a roll. “And Hermione’s done four feet seven inches and her writing’s tiny.”

The complaint about Snape's essay here is not that 12 inches is too long. The complaint is about the total amount of homework they've been assigned in such a short period of time. From later in the chapter:

“D’you realize how much homework we’ve got already? Binns set us a foot-and-a-half-long essay on giant wars, Snape wants a foot on the use of moonstones, and now we’ve got a month’s dream diary from Trelawney! Fred and George weren’t wrong about O.W.L. year, were they? That Umbridge woman had better not give us any...”

And from the subsequent chapter:

Now panicking slightly about the amount of homework they had to do, Harry and Ron spent their lunch hour in the library looking up the uses of moonstones in potion-making. Still angry about Ron’s slur on her woolly hats, Hermione did not join them. By the time they reached Care of Magical Creatures in the afternoon, Harry’s head was aching again.

He swallowed a mouthful of potato and added, “I hope she doesn’t keep me too long this evening. You realize we’ve got to write three essays, practice Vanishing Spells for McGonagall, work out a countercharm for Flitwick, finish the bowtruckle drawing, and start that stupid dream diary for Trelawney?”

It seems clear that a 12 inch essay is not the issue. They have done those in the past. What they are not used to is having significant homework for so many different classes all at once right at the beginning of the term. They had been warned that O.W.L. year would be hard, and they are only now fully appreciating that.