Still not clear, and I believe the previous writer wasn't also... as to WHY Not QuantA. We know that a pronoun is a word that takes the place of a noun. ex: Erik is strong, he is very strong. So how is Quanto=When/How Long a pronoun? It's not taking the place of the noun - dinner - or... la cena.
I thought that words like Quanto/a... Quello/a... took the same ending as the noun in the sentence. So la cena...seems like QuantA would take an A.
Can we have it explained a bit more? According to your other example ... quanto tempo dura la cena... It still isn't taking an A ending? That's confusing to us who remember the o/a... ending rules. Grazie