A Song of Ice and Fire author George R.R. Martin has said he's written 1,100 pages of next book The Winds of Winter, the same amount he'd written as of December 2022, while also revealing there are eight Game of Thrones spin-off shows being worked on.
That assumes that you write a book in a totally linear fashion instead of in various different threads and chapters and then weaves them together later. Writing a book from page 1 to the last page without changing or writing anything in between would be unusual. If he knew for sure that the first half of the book was what he wanted and it wouldn’t change at all as he wrote the second half, then yeah he could do that but he’d have to be very confident in where he was going already, in which case he’d probably just have it all done.
That assumes that you write a book in a totally linear fashion instead of in various different threads and chapters and then weaves them together later. Writing a book from page 1 to the last page without changing or writing anything in between would be unusual. If he knew for sure that the first half of the book was what he wanted and it wouldn’t change at all as he wrote the second half, then yeah he could do that but he’d have to be very confident in where he was going already, in which case he’d probably just have it all done.