...is not to do it until you have to, but do it as soon as you notice certain things:

Like water draining quickly out the bottom of the pot, or the plant's apparent size being the same or 'larger' than the pot underneath it. Yellowing leaves/drooping for no reason can also be a sign of a plant being rootbound.

Wait until the soil in the pot has almost dried out, and then remove the whole rootmass/dirt to take a look. If you can see roots 'coating' the outside of the dirt mass, it's time to repot.

Your plants look fine (if a bit tiny) for their containers, but move at least one of the two still in the same pot to its own container. Will save some trouble later. They should be fine, and as soon as they 'notice' the space that their roots now have to move into, you will notice a large growth spurt

Also, try to repot a few times, rather than "just once", to help build up a dense rootball. Going from a small container to a too-large one will not let the plant build up a dense enough root system.

Or so I've Heard