I think it depends on your kitten. If she is starting to get "plump" then I would switch to adult food now, if she is slim and generally healthy, then I agree with Sharky and would feed kitten food for the first year.
Unless its a larger breed (maine coon, ragdoll, etc.) I would not keep kittens on kitten food past 5-6 months old. Most of mine have been small size breeds (rexes) and if they ate kitten food for a year, they'd be very fat cats. Charlie was switched to adult foods at 5 months old with no problems in growth etc. He still is maturing but at a slower rate then the first 6-7 months.
So you can switch to adult foods any time now - just do it gradually if you are feeding kitten/adult dry - mix the 2 together and over about a week or two put more adult then the kitten.
Thanks for the replies. I don't know what breed she is, I call her a mini Maine Coon bc of her coat. I had her weighed 3 weeks ago & she was 4kg or just under 9 pound. I wouldn't call her overweight. Sometimes she looks a little round in the tummy, mainly after eating or if she hasn't had much exercise.
My baby Sophia will be 10 months old next month, but she's been on "big girl food" since she was 6 months old, only because it's hard to feed her separate food from my other 2 adult cats, as she tends to finish her food, then gravitate to the other 2 bowls of almost-empty food and finish them up.