I wouldn't worry about the cueing becoming meaningless. A cue means, either, "you're peeing! This is the sound mommy makes for pee!" OR "This is a good time/place to go potty." I would cue when you get her in the potty place, and then if she does go, cue WHILE she's peeing. That way you get both "this is a good place to pee" and "you're peeing!" all in one little potty op!
Your DD is still very little. I would probably wait 30-60 seconds, and then re-diaper her. If you're sure she needs to poop or pee or something, then by all means, offer longer, but if you're just using timing, then a minute should be plenty.
Does it mean she doesn't have to go if she doesn't respond to the cue? Eeh, hard to say. Maybe she doesn't know she has to go, maybe she's uncomfortable or cold or not relaxing, it could be any number of things. DS is two, and he's been ABLE to pee on cue since he was probably 6 weeks old. Does that mean he always does if he has to? Definately not. Sometimes it takes a little convincing and patience.
Sounds like you're doing great! Keep it up!