Many breeders find that about 18 days is a good time to take the box away. But you can take your cues from the kits. If they are in and out all the time, it is time! It can be as early as 2 weeks, as late as 3 weeks, sometimes a bit more if it's really cold or the kits are behind or compromised in some way.
Taking the box out is as soon as possible is a good idea because those babies are using it as a potty box, and even if you clean it frequently, it will be pretty unhygienic. And who wants to clean nest boxes every day or two?
Usually the kits will clump together in a huddle to sleep like they did in the nest box, just without the box. I do find that giving them a resting board (the slotted ones stay cleanest but a wooden board will do too, it just needs to be changed pretty often when it gets soiled) is nice because their little paws still poke through the 1/2" x 1" cage wire for a while.
Taking the box out is as soon as possible gives the mom and kits more room to move around; it also allows you time to clean, disinfect and air out the box before its next use.