These days, never. I have the support of multiple monitors if I need to reference some particularly difficult code. Modern hard drives aren't going to blink at me storing code that's even several thousand lines long. Printing code IMHO is wasteful and generally useless but that's just me.

That said when I was first learning up to develop/program I thought nothing of printing 20 pages of code, so I suppose there might be a time and a place for it but I just find these days there isn't much use for it.