I had originally thought that the seasons were extended due to The Doom, which was 400 - 500 years ago. However, with The Long Night lasting a generation and being 8000 years ago, and the source of the season shift being "magic" (as explained in a related question), has there ever been a time that the seasons were normal? Is there any reference in the books to three month seasons?
There were possibly regular seasons in ancient history, even further back than the Age of Heros. Writing and records were rare even back then and most of what we know from that time is legends passed on by story.
The Maesters have looked into the matter but have not been able to conclude anything as there is just not enough information from that long ago...
Though the Citadel has long sought to learn the manner by which it may predict the length and change of seasons, all efforts have been confounded. Septon Barth appeared to argue, in a fragmentary treatise, that the inconstancy of the seasons was a matter of magical art rather than trustworthy knowledge. Maester Nicol's The Measure of the Days—otherwise a laudable work containing much of use—seems influenced by this argument. Based upon his work on the movement of stars in the firmament, Nicol argues unconvincingly that the seasons might once have been of a regular length, determined solely by the way in which the globe faces the sun in its heavenly course. The notion behind it seems true enough—that the lengthening and shortening of days, if more regular, would have led to more regular seasons—but he could find no evidence that such was ever the case, beyond the most ancient of tales.
The World of Ice and Fire - Ancient History: The Long Night
So to answer your title question: The seasons may have been a normal length at one time but it is prior to recorded history.
We don't have any relative information yet. So far in both the books and the show, there wasn't ever a mention that the seasons were normal (as we perceive them). Each season lasts many years and everyone considers it normal to have decade long summers and winters.
If I remember correctly, GRRM said at an interview of his that some of this kind of 'magic' season changing will be revealed at the end of the series.
I asked "Is magic coming back into the world because there are dragons, or are dragons coming back into the world because there's magic?"
George said, "Yes. Hmm, there's excellent cheese on that pizza!" (in combination with some of the stuff he said on a panel this morning, I take it to mean that the seasons, winter and summer, are magical in nature, and he's going to reveal what it's all about eventually, but not yet.)
It’s been a popular topic on the A Song of Ice and Fire forums, this whole matter of what causes the weird seasons. Suggested theories have ranged as far as suggesting dark planets in the near vicinity, perhaps a binary star, and more. But it’s rather fruitless; the author is prosaic on the topic and has provided the direct answer: it’s magic, trying to figure out a scientific, realistic explanation is bound to fail.