I’m sorry if this has been answered before, but how soon after being cursed does a little’s knack start up? I know that Brothers Together said his knack came through after a few years…

It depends on the person, and how strong the curse is when its put on them (by the time Celeste curses Sam, she’s refined her technique, but some of the first borrowers to be cursed exhibited signs of their new abilities within days or months because of how overpowering the grace was).

Sam and Dean generally start to exhibit real signs of their knack around 2 – 3 years after they’ve been cursed. It starts out as weak twinges, and then grows in strength. Dean’s uncanny ability to always stumble into food becomes an ability that lets him track down anything he needs. Sam thinks he’s just good at predicting when a human’s about to walk into the room, but instead he’s feeling that their attention is coming closer to him.

A borrower that is born instead of cursed will have that ability their entire life. Whether they exhibit will depend on their specific situation. Only 1 or 2 borrowers out of 100 will actually show signs of their ability, the other 98 go on not realizing they have an ability at all, much like Walt Watch did until the moment he needed it most. 

Of course, the borrowers that the story tends to focus on are those with their special abilities, so we’ll be seeing quite a few new knacks as the stories progress!