My baby stopped taking the dummy after a month and I had to find a new one that she will like more cause otherwise she would be on my boobs all day. I got a trial pack from Bibs, they do 3 shapes and my baby took one after a few days of trying and she loves it now. She usually takes it before a nap and for the nap, sometimes at night she cries for it too, also for car trips. What I did is I was dipping it in a cup of hot, preboiled water and giving it to her, I would hold my hand on her chin so it feels like a breast to her so a bigger chance she will take it. Once you have a dummy your baby is happy with, I would change it for a bigger size when baby is ready. Every baby is different so she may not want a bigger size exactly at 6 month.
My baby kept spitting dummy out but I new he liked it to settle so I got a size up and he finally kept it in his mouth n is much more content my baby is also in 6-9 month clothing. My baby on uses his dummy as a comfort to get to sleep n then it falls put or his mouth when he is properly asleep. But if u find that dummies aren't really helpful and don't do much for ur baby then u could probably just stop using it x