Well, with my current project, I'm somewhat limiting how my characters respond. There is more unique dialogue for my characters than for random NPCs. Though, there are variables which can turn on new dialogue options or change how dialogue would normally be given. It's a fairly annoyingly complicated thing that I've been trying to minimize as much as possible simply because coding 14+ pages on each NPC gets to be ridiculous to cover each character in the party, character disposition, events completed, etcetera. If it were easier to just check for specific things and have those show up without a maddening amount of coding, I'd do that instead. Most of what I'm doing is adding branching conditions in the pages that if certain characters are present or what-have-you, some dialogue changes and other dialogue is added, etcetera. It seems somewhat simplistic until you realize that I have to do this for 8 other characters, two different dispositions of the main character, and any other variable you would take into account (like previous quests finished or started, etcetera).
Though, honestly, how often do people click the same NPC more than that one time? A player will generally click an NPC either once or until all the dialogue with that NPC has been run. The most I've ever clicked an NPC was probably in Secret of Evermore where there was a single NPC who was spouting off lines about "the end of days" and then started telling everyone that they were all characters in a video game. There was an option partway through that dialogue in which the "powers that be", namely the player, could turn the NPC into a few different things and would end his rant. If you skipped the box though, you could finish his dialogue and he'd give you a reward for listening to the whole 5 minute dialogue or whatever it ended up being. I think it was something like 8-14 dialogue boxes for him. Longest I ever saw, and was worth it with the joke payoff as well as the option to listen to his rant.
I don't think it matters HOW MUCH your NPCs have to say... per sé. I think it matters WHAT those NPCs say. Quality over quantity. I don't recall the scene with the doomsayer in Secret of Evermore 'cause it was long. I remember it, 'cause it was somewhat funny and interesting and was a neat little thing for the programmers to throw in there. Could I tell you what other NPCs say in the game? Not likely. Most RPGs are like that. If you're going for memorable characters, then your NPCs need to spout somewhat interesting things to the player so they remember. If the NPCs are just there to add flavor to the environment and help nudge you in the right direction, then you don't need anything other than a single dialogue option.
