No spoilers please.
I am currently reading Fables in trade paperback form. When do I need to start (planning for and) buying and reading Jack of Fables?
I.E when does the spin-off begin in relation to Fables?
The "Great Fables Crossover" is between Fables #83–85, Jack of Fables #33–35 putting a spit around Fables #50.
I would suggest picking up "Jack of Fables v1 - The (Nearly) Great Escape" just after Fables: Wolves which collects Fables #48-51.
From Wikipedia:
It shows the adventures of Jack Horner that take place after his exile from Fabletown in the Fables story-arc Jack Be Nimble (Fables #34 and #35).