Digital insulation meter measurement limits
Remember Ohm's law:

500V across less than 1.5M gives greater than 0.33mA and over 0.17 watts

1000V across less than 5M gives greater than 0.2mA and over 0.2 watts

The watts are about the same in both cases, so their 10sec limit may be to prevent internal heating of the meter's own HV supply ...but more likely it's for fire hazard.   If the >0.2 watts is being drawn by a corona or an arcing point-fault in your cable insulation, then 10sec of a >fifth-watt heating may cause permanent damage, even flames.

If the cable leakage draws less than 0.167 watt from the meter's supply, then you're within their published V-I limits, and the 10sec doesn't apply.

It's a good idea to perform a one-second test, then if you find >1.5M or >5M of insulation leakage, go and find the break in the insulation.