Page "Cartan–Karlhede algorithm" Paragraph 10

An important unsolved problem is to better predict how many differentiations are really necessary for spacetimes having various properties.

For example, somewhere two and five differentiations, at most, are required to compare any two Petrov III vacuum solutions.

Overall, it seems to safe to say that at most six differentiations are required to compare any two spacetime models likely to arise in general relativity.