I have seen doxycycline dosage for chickens as 20-25 mg per Kg (each 2.2 pounds of weight,) as a daily dosage. So if you gave 1/4 tablet daily, that would probably be close. You could then divide that 1/4 tablet into 2 divided doses to be given in the morning and in the evening. I would mix it into a treat they will surely eat. Can you post a picture of the label of your product?
Edited to add that you should 1/8 a tablet daily, not 1/4. That would be about 10-12 mg per pound daily.
5-7 days. Sorry for my lack of math skills. I hope that she improves.