derivation-length

In the correct grammar of the previous question, what is the length of the derivation (number of steps starring from S) to generate the string $${a^l}{b^m}$$ with $$l \ne m$$?

If $$G$$ is a context-free grammar and $$w$$ is a string of length $$n$$ in $$L(G),$$ how long is a derivation of $$w$$ in $$G,$$ if $$G$$ is Chomsky normal form?