How long is a piece of string? Yes, insulation resistance can go all over the shop with sheathed elements. I agree with the 0.3MΩ as an attainable limit but it's a bit of a last resort. Good elements that have been baked out should be able to do better than that.
the cooker tested 0.15M ohms N - E
As
@timhoward suggests, N-E (or L-E) is not an ideal test for an appliance, at least at 500V. Always IR test appliances as a PAT instrument would, with line and neutral connected together, to avoid any of the test voltage appearing between L & N if there is leakage from one side to earth only. In many situations it wouldn't matter as there is resistive load between L & N to collapse the test voltage, but if the only load is a small electronic control / timer module, test voltage escaping via leakage could cause damage.
Is 0.15MΩ dangerous though? E.g. by causing earth leakage? Let's suppose the elements are switched / controlled in the line side only and the leakage is entirely from the elements. The element resistance itself is absolutely negligible to the IR test, so by measuring to the neutral you will read all the leakage from both the line and neutral ends of the elements in parallel (it's likely to be at the ends.) If it's the same at both ends, we might suppose there's 0.3MΩ at the line ends and 0.3MΩ at the neutral. Connected to 230V with all elements on, that would result in an earth leakage of 230/0.3M = 0.77mA so really quite a trivial amount if the IR remained at that level.
But this brings us to the crux of IR testing: A marginally low reading doesn't usually cause danger in itself through earth leakage, but as a diagnostic it points towards a defect that might become more dangerous over time, or be dangerous in other ways (such as rodent damage to a cable that doesn't leak much to earth but has exposed live copper.) So then we have to consider whether the 0.15MΩ is suggestive of such a defect in a cooker. If some elements haven't been used for a while and have collected some moisture, then possibly not. If they have all been in regular and recent use then that justification doesn't really apply, it's a bit too low and unless it recovered to a high value after brief heating, I would consider it faulty. We've no proof that the low IR is caused by the elements - it could be charred wiring that has been pressed against a hot surface where it shouldn't be - but 98% of the time it will turn out to be the elements.
So, it's a bit of an awkward one to argue with a customer. The resistance reading itself is on the cusp in the event that the elements were not all baked out before testing. Then you have to consider the distinction between technical failure, potentially dangerous and actually dangerous. The best one could do is point at the 0.3MΩ threshold and suggest that by 'potentially dangerous' what you mean is that it requires baking and re-testing, and in the event that it doesn't improve at least beyond that threshold, the cause is by no means certainly the elements and requires investigation before you can declare it safe.
Back in early 1990s when I was overseeing hundreds of schools having their first ever PAT / ISITEE, the Baby Belling portable cookers that hadn't been used for ages failed consistently due to moisture. Most schools wanted to keep them even if they only saw occasional use, and we found that by buying new elements in bulk and getting a hefty discount, it was quicker and cheaper to change them on spec than to fiddle around testing, baking, retesting. They used to send the cookers back to the workshop and I got very adept at changing them. Even while offering an excellent rate for the service 40% lower than any competitor, if I did them myself I could earn more per hour turning those cookers around than pretty much anything else I've ever done on the tools.
