Thread: changing food

I am in the process of changing the girls onto adult food, however their poo has now gone runny. How long would you give before you decide the new food is not for them and try another brand? This has been over a two week period, I have mixed the new Sanabelle grand with their Royal Canin kitten food to wean them onto it. I'm getting a bit fed up with smelly runny poos now .