It really just depends.
Sometimes the cards have a lot to say. Let's say, just for example, I'm going a three-card spread and I draw The Lovers, the Two of Cups and Temperance. There's going to be a lot I can say about that reading, because the cards would mesh together well and enhance one another seamlessly. But if it was a more choppy reading...like, say...The Devil, Nine of Cups, 4 of Wands, I would probably talk less and the querent would probably talk more, just because of my reading style. I tend to ask questions a lot and have the sitter fill in a lot of blanks and help make the reading more personal and more fitting, so when I'm having trouble stringing the cards together, I talk less and my sitter talks more.
That being said, sometimes less really is more. If you make a reading wordy, your sitter is going to have a lot more 'stuff' to try to remember, which means that most of the more important things are going to get lost of overlooked. If you keep things short and concise and focused, your sitter is going to leave knowing exactly what you said and what was important.
Maybe you could try reading for yourself, but doing the reading out loud. You know, pretend that you're reading for an invisible person (an imaginary friend, if you will). That would get you used to talking about the cards out loud, and get you thinking on the spot. I actually read for stuffed animals, my cats, one of the dogs, inanimate objects, etc. regularly, just to keep myself thinking and looking at the cards in a new way.