I'd set a timer. Tell her when the timer goes off- done.
You gotta remember its like anything in parenting. You set the boundaries and gently guild her how to manage the boundaries. When I weaned, we simply had no time to nurse, I said and we went to the zoo, then sea world, then the beach, each day. Busy busy busy. And it only took a few days of distraction to completely wean. I had fertility issues and was hoping I could get my cycles back. Nope. :/ lol
BFing, CDing, all natural mama to three boys! Certified lactation Counselor