We actively see owls arrive at various times throughout the day at both Hogwarts, as well as peoples homes.
Examples of mail that arrived other times.
- Charlie Weasley - Sorcerer's Stone (Midnight)
The clock on the wall had just chimed midnight when the portrait hole burst open. Ron appeared out of nowhere as he pulled off Harry's invisibility cloak.
There was a tap on the dark window. "It's Hedwig!" said Harry, hurrying to let her in. "She'll have Charlie's answer!"
- Ministry of Magic - Chamber of Secrets(approx 9-10 pm)
Aunt Petunia was just passing around a box of after-dinner mints when a huge barn owl swooped through the dining room window, dropped a letter on Mrs. Mason's head.
- Hedwig - The Prisoner of Azkaban (1AM)
Silhouetted against the golden moon, and growing larger every moment, was a large, strangely lopsided creature, and it was flapping in Harry's direction.
Examples of official mail that arrives at breakfast.
- Hogwarts acceptance letters - Chamber of Secrets
Harry heard from Hogwarts one sunny morning about a week after he had arrived at the Burrow. "Letters from school," said Mr. Weasley, passing Harry and Ron identical envelopes of yellowish parchment, addressed in green ink. "Dumbledore already knows you're here, Harry - doesn't miss a trick, that man.
- Daily Prophet - Sorcerer's Stone
The owl then fluttered onto the floor and began to attack Hagrid's coat. "Don't do that." Harry tried to wave the owl out of the way, but it snapped its beak fiercely at him and carried on savaging the coat. "Hagrid!" said Harry loudly. "There's an owl." "Pay him," Hagrid grunted into the sofa. "What?" "He wants payin' fer deliverin' the paper.
- Packages from mail order companies - Sorcerer's Stone
As the owls flooded into the Great Hall as usual, everyone's attention was caught at once by a long, thin package carried by six large screech owls.
So While it appears while typically official mail tends to arrive during the breakfast window, mail does not have a required time to arrive.
A side note: it does appear that usually Hogwarts halts package delivery till breakfast, as seen by this note from McGonagall that was written and placed on the package before breakfast.
Harry ripped open the letter first, which was lucky, because it said:
DO NOT OPEN THE PARCEL AT THE TABLE.
It contains your new Nimbus Two Thousand, but I don't want everybody
knowing you've got a broomstick or they'll all want one. Oliver Wood
will meet you tonight on the Quidditch field at seven o'clock for
your first training session. -Professor McGonagall (Emphasis mine)
UPDATE- from Order of the Phoenix.
Percy sends a letter that arrives at 11 pm, and specifically states he sent this letter to arrive after the usual morning post. Whether this was simply by sending it later, OR but telling his owl to not join the usual post.
But I want to give you more than congratulations, Ron, I want to give you some advice, which is why I am sending this at night rather than by the usual morning post. Hopefully, you will be able 'o read this away from prying eyes and avoid awkward questions.