Perhaps the more interesting question that emerges from the posted question is, When did condescension acquire the meaning of—as the American Heritage Dictionary of the English Language, fifth edition (2010) puts it—
Patronizingly superior behavior or attitude.
There is no such meaning in Samuel Johnson, A Dictionary of the English Language (1755), which offers this entry for the word:
CONDESCENSION. s. {from condescend} Voluntary humiliation ; descent from superiority ; voluntary submission to equality with inferiours. [Examples:] It forbids pride and ambition, and vain glory ; and commands humility and modesty, and condescension to others. Tillotson. | Courtesy and condescension is an happy quality, which never fails to make its way into the good opinion, and into the very heart, and allays the envy which always attends a high station. Atterbury's Sermons. | Raphael, amidst his tenderness, shews such a dignity and condescension in all his behaviour, as are suitable to a superiour nature. Addison's Spectator, No. 273.
The definition of condescension in Todd's 1818 revision of Johnson's dictionary, published five years after the first publication of Pride and Prejudice, retains Johnson's wording from 1755 to the letter.
Nor does Noah Webster, An American Dictionary of the English Language (1828) break any new ground toward arrogance or patronizing behavior in a negative sense:
CONDESCENSION, n. Voluntary descent from rank, dignity or just claims; relinquishment of of strict right; submission to inferiors in granting requests or performing acts which strict justice does not require. Hence, courtesy.
The same entry appears in the 1847 edition of Webster's dictionary and virtually the same one appears in the 1864 edition, as well.
In Webster's International Dictionary (1890), however, a new element cautiously emerges:
Condescension, n. The act of condescending; voluntary descent from one's rank or dignity in intercourse with an inferior; courtesy to inferiors.
The definition of condescend takes on a new tone, too:
Condescend, v. i. 1. To stoop or descend; to let one's self down; to yield; to submit; to waive the privileges of rank or dignity; to accommodate one's self to an inferior. ... Often used ironically, implying an assumption of superiority. [Example:] Those who thought they were honoring me by condescending to address a few words to me. F.W. Robinson.
Even more striking is the discussion in Webster's New International Dictionary (1909) of the difference in sense between condescend, deign, and vouchsafe:
Condescend implies a courteous or patronizing waiving of real or assumed superiority ; as, "No beggar ever felt him condescend, no prince presume" ([James Russell] Lowell); "They insult me with their insolent condescension" (Byron).
The Byron quotation is particularly striking, since it comes from a letter to Mr. Murray, dated April 6, 1819:
Besides I mean to write my best work in Italian, and it will take me nine years more thoroughly to master the language ; and then if my fancy exists, and I exist too, I ill try what I can do really. As to the estimation of the English which you talk of, let them calculate what it is worth, before they insult me with their insolent condescension.
I have not written for their pleasure. If they are pleased, it is that they chose to be so ; I have never flattered their opinions, nor their pride ; nor will I. Neither will I make 'Ladies' books' 'al dilettar le femine e la plebe.' I have written from the fulness of my mind, from passion, from impulse, from many motives, but not for their 'sweet voices.'
So on the one hand we have mainstream dictionaries taking no formal notice of any undercurrent of presumptuous superiority in the idea of condescension until around 1890; but on the other we have a letter by the celebrated Byron in 1819 (and published by 1831) in which condescension fairly drips with unwarranted patronization.
It is quite possible that Jane Austen had never heard condescension used with an ironical or hostile implication at the time that she wrote Pride and Prejudice; but nevertheless, the seeds of what amounts to a 180-degree reversal of the original "courteous and voluntarily egalitarian" sense of the word may already have sprouted in some parts of the English-speaking world.