When its big enough, imo that is just before it fills the pot with roots.
I like my plants (any plant) to have a little original soil with it in the new pot to grow in. But there is no problem to start growing directly in a big pot either, as long as you keep it humid.
Traansplant in humid soil, let it dry out before watering, first 24hrs in shade/little light to stimulate root growth.