How low an insulation resistance is too low for a residential electrical system?
UK perspective: when I trained as an electrician (early 80s) the pass/fail for an insulation test on an installation was 1 megohm. I just checked the 17th Edition and it still is. That's with the proviso that if you get a result that low, or basically not very high, you should check what's causing it. This is also with the proviso that you're testing it at twice the rated voltage, so on 240V (as it was then) that meant using a 500VDC insulation tester. A low voltage test (e.g. with a multimeter) is not considered valid, whatever reading you get.