It's possible that John Dashwood was by nature easily persuaded by Fanny who repeatedly (in Ch 2) used their own son to manipulate his heart. Consider: "to take three thousand pounds from their dear little BOY" and "begging you to give away half your fortune from your own CHILD" and then "if it indeed could be restored to our little BOY."

After a while, John could've possibly come to his senses, and felt the intense guilt of robbing his own heir of such a fortune. What could a father do? After all, Fanny's arguments made a whole lot of sense...

I do believe that if Fanny kept talking, her lips of honey would have eventually persuaded John into giving them nothing at all. After all, as Fanny pointed out in Ch 2: "To be sure it is: and, indeed, it strikes me that they can want no addition at all. They will have ten thousand pounds divided amongst them. If they marry, they will be sure of doing well; and if they do not, they may all live very comfortably together on the interest of ten thousand pounds." So why give them anything at all :-)

As for time, I believe this was a long conversation between John & Fanny, but from the text, I'm inclined to believe it was not agonized over days, but only a matter of hours. Unfortunately I could find no proof of this as I reread the chapter over and over again... I suppose we have our own thoughts. I was imagining that the conversation was very hush-hush. Perhaps they were both in their bed chamber mulling over the events of the day.

I think that to begin with, John wasn't entire sold out on his own decision, in spite of his thinking he would not repent of the decision. I suspect he held on to his money tightly, and it was the pain of losing his father that brought about his emotional response of wanting to surrender £3000 of his own money to do it right by his father.

When people are in an emotional state, they sometimes make decisions that they later back out of when reason is pressured onto their thinking. And in John's case, Fanny has the Newton on him. "The argument was irresistible" (ch 2).

As for the days of creation, according to the text, it only took six days and not seven, since God rested on the seventh day :-)