When should kitty stop taking formula?

I already posted when I was given a 3 week old kitten who wouldn't stop crying. She's fine now, I bought her formula and she's always playing with my older cat. My question is when should she start to eat cat food? She takes almost two bottles of formula per day, and it doesn't seem to fill her. I've tried mixing solid kitty food with the formula and wet food, but no luck. it there anything I could do to make her eat? Or, is she still too young to lay off the bottle?

ETA Forgot to add she's about five weeks old now.

At five weeks she's ready to move towards solids so if what you're offering isn't enticing her, maybe something else will. Mine never would take food mixed with formula--guess to him milk was for the dropper and not the dish. I had good luck starting with a chicken baby food (the first steps kind without spices). Once he was used to getting something from a plate he took dry food soaked in water and then eventually the canned stuff with water mushed in.

Even when he started moving over, he stuck with the formula for a while longer, which was needed since without a role model he didn't catch on to lapping up water until fairly late.