What is a "slap week"?

The term is used in Harper Lee's Go Set A Watchman, in the following passage. The context is that Jean Louise has been preparing for the big dance at the high school; the referent for "them" in the first paragraph, although irrelevant to the meaning of the expression, is a set of falsies:

As Jean Louise handed them over, a sudden thought rooted her to the spot. “Oh golly,” she whispered.

“What’s the matter now?” said Calpurnia. “You’ve been fixin’ for this a slap week. What did you forget?”

“Cal, I don’t think I know how to dance.”

Slap appears to be a regionalism used in the American South meaning the entirety of something, which can be used to emphasize a magnitude of difficulty, achievement, and so on. I have not found it in dialect or slang dictionaries, but it does turn up online in various places:

It was a slap year before it quit hurting.[1]

I make sure every product page has a unique title and meta description. Took me a slap month to write them for 2500 products on one site, but I did it. [2]

It took him a slap hour to total up the numbers.[3]

Remember, these are sneaky, underhanded, devious people who kept the Barnes flag deal under wraps for a slap YEAR.[4]

I've been waiting on this for a slap hour! [5]

It took me a slap month to finish reads to me much as it took me an entire month to finish or perhaps it took me literally a month to finish, and indeed it reads as such when it appears in Harper Lee's earlier work, To Kill a Mockingbird, spoken by Tom Robinson quoting Mayella Ewell in testimony:

Mr. Finch, I was wonderin' why it was so quiet like, an' it come to me that there weren't a chile on the place, not a one of 'em, and I said Miss Mayella, where the chillun? … [A]n' she says — she was laughin', sort of — she says they all gone to town to get ice creams. She says, 'took me a slap year to save seb'm nickels, but I done it. They all gone to town.'

Examples of time were the easiest to find in a web search, but they are far from the exclusive or dominant use. For example, Southern Living magazine notes the expression worn slap out meaning a state of total exhaustion—

a physical and mental state a few degrees past weary and just this side of dog-tired.

To be tore slap up is to be seriously damaged, injured, or impaired, as torn up might be used in a more standard English; a miner in a William W. Johnston novel had one ear tore slap off.

The OED does not list this sense under any definition of slap, but I could see it arising out of its adverbial senses of straight or directly or suddenly. A slap day is one without interruption, after all. OED examples of this meaning are attested from 1829:

I, and my Noah's Ark, lay slap in the way.