Concerning Karna, her conception did not occur the normal way, rather it was done magically by Surya, as described in this chapter of the Vana Parva of the Mahabharata:
And that ranger of the skies, that enemy of Swarbhanu, with soul absorbed in Yoga, entered into Kunti, and touched her on the navel. At this, that damsel, on account of Surya's energy, became stupefied. And that reverend lady then fell down on her bed, deprived of her senses. Surya then addressed her, saying, 'I will now depart, O thou of graceful hips! Thou shalt bring forth a son who will become the foremost of all wielders of weapons. At the same time thou shalt remain a virgin.' ... Then, O foremost of kings, as the highly effulgent Surya was about to depart, that girl bashfully said unto him, 'So be it!' And it was thus that the daughter of king Kuntibhoja, importuned by Surya, had after soliciting a son from him, fallen down stupefied on that excellent bed, like a broken creeper. And it was thus that deity of fierce rays, stupefying her, entered into her by virtue of Yoga power, and placed his own self within her womb. The deity, however, did not sully her by deflowering her in the flesh. And after Surya had gone away, that girl regained her consciousness.
But her pregnancy and delivery were normal as described in the next chapter of the Vana Parva:
It was, O lord of earth, on the first day of the lighted fortnight during the tenth month of the year that Pritha conceived a son like the lord himself of the stars in the firmament. And that damsel of excellent hips from fear of her friends, concealed her conception, so that no one knew her condition. And as the damsel lived entirely in the apartments assigned to the maidens and carefully concealed her condition, no one except her nurse knew the truth. And in due time that beauteous maiden, by the grace of deity, brought forth a son resembling a very god.
Concerning the Pandavas, this chapter of the Adi Parva of the Mahabharata says that their births were separated by one year from one another:
O king, after all the children were born the Rishis dwelling on the mountain of a hundred peaks uttering blessings on them and affectionately performing the first rites of birth, bestowed appellations on them. The eldest of Kunti's children was called Yudhishthira, the second Bhimasena, and the third Arjuna, and of Madri's sons, the first-born of the twins was called Nakula and the next Sahadeva. And those foremost sons born at an interval of one year after one another, looked like an embodied period of five years.
And since the previous chapter of the Adi Parva describes Kunti using the mantra to summon the next god as soon as the current child is born, I think it's pretty clear that the pregnancies took the normal length of time.
As far as the death of Pandu and Madri, it happened a significant time after all five Pandavas had been born, as described in another chapter of the Adi Parva:
Beholding his five handsome sons growing up before him in that great forest on the charming mountain slope, Pandu felt the last might of his arms revive once more. One day in the season of spring which maddens every creature the king accompanied by his wife (Madri), began to rove in the woods where every tree had put forth new blossoms.... And, O thou of Kuru's race unrestrained by the fear of (the Rishi's) curse and impelled by fate, the monarch, overpowered by passion, forcibly sought the embraces of Madri, as if he wished to put an end to his own life. His reason, thus beguiled by the great Destroyer himself by intoxicating his senses, was itself lost with his life. And the Kuru king Pandu, of virtuous soul, thus succumbed to the inevitable influence of Time, while united in intercourse with his wife.
And the Pandavas were old enough to carry out funeral proceedings for Pandu, as described in this chapter of the Adi Parva:
Thus weeping, Bhishma, Vidura, Dhritarashtra, the Pandavas and the Kuru ladies, all performed the watery ceremony of the king. And when all this was over, the people, themselves filled with sorrow, began to console the bereaved sons of Pandu. And the Pandavas with their friends began to sleep on the ground. Seeing this the Brahmanas and the other citizens also renounced their beds. Young and old, all the citizens grieved on account of the sons of king Pandu, and passed twelve days in mourning with the weeping Pandavas.