Interesting question, but hard to say.....It would depend to a large degree how familiar I was with the airfield. I usually fly a VFR only cub, no DI or AH, and get quite edgy cross country at less than 3 km visibility even at cub speeds. (No nav aids, no GPS) I would certainly be more at ease in the circuit at that vis, but I don't often do circuit bashing for the sake of it, as I get plenty of approaches when tugging. And of course glider pilots don't like flying inside a milk bottle either, so I tend not to have to do poor visibility approaches. I have a lapsed IMC and about 50 hours real instrument flying, if that helps to classify me, lapsed CPL, now flying on PPL.
So, to answer as best I may, about 1 km which is a bit over the length of our grass runway would take me to my discomfort zone. I would expect to get in, but not to want to make a habit of it.
If I had an NDB available I doubt if it would make a lot of difference at my home base, but it would be nice to have at an unfamiliar airfield, at least to point me at the runway. Runway lighting would help too!