There's no real set amount that's required. It sounds like you're trying everything. It may be, like I said, that she's just not emptying her bladder each time she goes because she knows she will get to go out again fairly soon. What you can do is try leaving ice in her bowl instead. Sometimes, like with my dogs, if there's water there, they feel the need to finish it...no clue why they just do so I just keep it outside and give the ice inside. There are medical reasons for it also. If she's incontinent she would be doing this. Also sometimes if they've just been spayed, sometimes the bladder is manipulated and will temporarily cause this. Sorry I wish I could be more help but hopefully I answered want you need.
Good Luck!
Let me know if you have any other questions
Good Luck &
Best Fishes