When do you take down your whelping box and move to a bigger pen?

I have a whelping box that I can take apart and add longer boards - I do that at about 3 weeks to allow them to potty at one end. They go out at 5 weeks

At 3 weeks when they start looking for a place to pee. I open the end of the whelping box, use short x-pen and put down papers. At this age they find the papers.

Both of your boxes sound ideal. I wish my whelping box could be extended or one side removed.

I prefer to keep them in my mudroom as long as I can so I have a 2ft extension for my 4x4 box and add a litter box when I set up the extension.

If/When they figure out how to get over the 18" sides, they go to a 6x6 kennel in the basement.