I think Darcy should have told Bingley the truth after he learned he was wrong about Jane's regard for Bingley. Bingley only abandoned Jane because he believed her indifferent at Darcy's word and I think he should have known the truth as soon as possible. I think withholding this truth is just as bad as keeping the information Jane was in town from him. (;D) I came up with five reasons it takes Darcy *five and a half to six months before correcting his error in separating Jane and Bingley.

The first is that even after he learned he had misjudged Jane’s regard for Bingley he did not believe he had been wrong in separating them due to her family’s impropriety. In April he leaves Rosings without a fiancé but otherwise thinking much the same as when he arrived.

The second is I think his desire for a Bingley-Georgiana match unknowingly colors his thinking.

"No," said Darcy, "I have made no such pretension. I have faults enough, but they are not, I hope, of understanding. My temper I dare not vouch for. It is, I believe, too little yielding -- certainly too little for the convenience of the world. I cannot forget the follies and vices of others so soon as I ought, nor their offences against myself. My feelings are not puffed about with every attempt to move them. My temper would perhaps be called resentful. My good opinion once lost is lost for ever." (Chapter 11)

"I cannot be so easily reconciled to myself. The recollection of what I then said -- of my conduct, my manners, my expressions during the whole of it -- is now, and has been many months, inexpressibly painful to me. Your reproof, so well applied, I shall never forget: 'Had you behaved in a more gentlemanlike manner.' Those were your words. You know not, you can scarcely conceive, how they have tortured me; though it was some time, I confess, before I was reasonable enough to allow their justice." (Chapter 48)

The third is I think it took time for Darcy to realize he was wrong to separate them, even longer than it took him to come to grips with his ungentlemanlike behavior during the proposal. I am not sure when he decides he was wrong to separate them—perhaps not until he meets Lizzy again at Pemberley in *August and then maybe only because he starts thinking of pursuing Lizzy again a half hour after meeting with her. His pursuing Jane’s sister makes his separating Jane and Bingley absurd. I think there is a good chance Darcy came to feel he had been wrong during the Pemberley chapters because otherwise I think he would have tried to discourage Bingley from returning to Netherfield and that does not seem the case at all.

The fourth reason is Lydia’s elopement derailed any other projects and ideas till she was saved.

"On the evening before my going to London," said he, "I made a confession to him which I believe I ought to have made long ago. I told him of all that had occurred to make my former interference in his affairs absurd and impertinent. His surprise was great. He had never had the slightest suspicion. I told him, moreover, that I believed myself mistaken in supposing, as I had done, that your sister was indifferent to him; and as I could easily perceive that his attachment to her was unabated, I felt no doubt of their happiness together." Elizabeth could not help smiling at his easy manner of directing his friend.

"Did you speak from your own observation," said she, "when you told him that my sister loved him, or merely from my information last spring?"

"From the former. I had narrowly observed her during the two visits which I had lately made her here, and I was convinced of her affection." (Chapter 58)

The fifth is Darcy needed to personally observe Bingley and Jane together before admitting anything to his friend. Bingley and Darcy return to Netherfield in *September and Darcy spends two visits observing them before he admits his impertinent and absurd interference. Perhaps this is where Darcy’s overdeveloped sense of responsibility applies, it seems he could not bring himself to advise Bingley on Lizzy’s assertions of Jane’s feelings alone and she does not seem offended by this. Lizzy and Darcy did not discuss Jane and Bingley at the Lambton inn or at Pemberley. I cannot really blame Darcy for wanting to advise Bingley on his own observations rather than Lizzy’s four month old intelligence (Chapter 34) because Jane’s feelings may have changed over the past five and a half months.

I would have thought Darcy might have seen Bingley’s attentiveness to Lizzy at the inn in Chapter 44 as a bit of a clue to Bingley’s continuing feelings for Jane. Lizzy is pleased Bingley remembers exactly the last time they were all together dancing at the Netherfield. I could not find in the text that Darcy does notice Bingley’s reaction to Lizzy or his skirting around the Jane issue; it might be Darcy is so focused on Lizzy that he does not see how Bingley is acting—after all Darcy is so focused on Lizzy he does not notice Georgiana’s embarrassment when Caroline hints about Wickham to Lizzy in Chapter 45. If not for Lydia’s elopement I think Bingley and Darcy might have returned to Netherfield and Darcy confessed earlier.

* See A Calendar for Pride and Prejudice by Ellen Moody in the P&P FAQ:
http://www.jimandellen.org/austen/p&p.calendar.html