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.
I don't have access to the whole DIN norms but I found a document citing DIN VDE 0701 (09.2000) and DIN VDE 0702 (06.2004).
Which contains a table:
Limit value: Applicable for:
0,3 MΩ class 1 with turned on heating elements
1,0 MΩ class 1
2,0 MΩ class 2 and non-earthed metal parts on class 1
0,25 MΩ class 3
So this is for the devices and not the house installation.
I found a reference to IEC 60364-6 (limits in chapter 61.3.3, table 6A) and IEC 61557-2 in this document which has:
Limit value: Test voltage: Voltage of the circuit:
0,5MΩ 250 VDC SELV / PELV
1,0MΩ 500 VDC up to 500 V including FELV
1,0MΩ 1000 VDC over 500 V
I'm not 100% sure if this also counts for the house installation, but the document made it sound like it does.
For solar panels I also found something in this document which references DIN EN 61646 and DIN IEC 61215:
The insulation resistance has to be larger than 40 MΩm². So a panel with 1 m² has to have at least 40 MΩ, one with 2 m² needs at least 20 MΩ.
This is the stuff I found, everything in German so my translation might not be correct and I certainly take no claim that this information is valid or even applicable outside of Germany.
In industrial manufacturing environments the rule of thumb is 1 Meg ohm is sufficient insulation per 1000 volts. I currently have a motor with less than 1/2 Meg ohm to ground, that has run 24 hours a day for over 5 years at 300 volts D.C. With no signs of failure.
For home purposes I would megger at the 500 volt scale and would consider anything over 1/2 megohm to be ok.