The general rule is that if they've gone longer than fourty minutes at the breast, then they're lollygagging around. So start with that.
Just start using the paci. She'll suck for as long as she feels the need to and then spit it out. If she needs to suck some more or suck later, she'll groan and wiggle and be happy sucking when you put it back in. If she's not happy taking it because she is used to getting mommy, just hang in there. After two or three days of only offering her your breast when you know she ought to be hungry and offering the paci at all other times, she'll be used to the paci. The key is consistency.
And don't worry yourself with, "Well, maybe she is hungry." Err on the side of caution. Give the paci, then give sleep a good try. You'll soon be able to tell when she's really truly hungry as opposed to tired.
My little girl, at that age, often wanted to suck when she had gas or indegestion. If I had put her on the breast I actually would have aggravated her indegestion and gas. A good burp or some time was all she needed and then she was ready for bed by then. Think about that when it's only been an hour or two since a feed.
Hope that helps!