I researched this
I ran into the same situation in my novel. I went to my home library and skimmed all my books. I could not find a single example of a speech lasting longer than a middle length paragraph. No long.
If you want to make it longer, add some short action tags. You know, the talker smiles, waves her hands, sit downs, grimaces, the listeners do likewise. Give the reader some relief.
You know how to use quotes in multiple spoken paragraphs right? It unbalanced them, because you don't use a ending quote at the end of the paragraph, but you do use a quote at the start. I hate that unbalance, another reason to end a speech at a paragraph.
My .02, not reading your story, that except in usual cases, 5-6 spoken sentences is the maximum advisable. If you go longer, it had better be gripping and intense, otherwise I guarantee your reader will simply skip over it.
Dumb example, if you have a crazy murderer lecturing the tied bad guys about how he's going to blow up the world, go for it, but have them struggling, reacting, making plans as he struts and gloats, break it up.
Unless your hero is named Hamlet, try to avoid it.
My main charecter is speaking, imparting knowledge that in the situation only he knows to two other charecters. At present this peice of dialogue is almost 600 words long and is uninterupted by the two women.
My question is this - is the dialogue too long?
How long is too long?