When should we apply for I 601A waiver?

You should file the I-130 immediately to shorten the time for her priority date to become current. It's currently taking at least 7 years for the F1 category. Only immediate relatives can apply for an I-601A provisional waiver. Since your daughter is already over the age of 21, she can no longer qualify as an immediate relative, even if you become a US citizen.

You will have to file an I130, get it approved and then wait till the priority date gets current to apply for I601 waiver.

You should file the I-130 visa petition for her immediately. That will ensure that she gets in line for visa eligibility. An I-601A is unfortunately unavailable to her as she is not an eligible relative to file the waiver before she leaves the country. These waivers can only be filed by people who are in the immediate relative category such as parents or minor(under 21) children of United States Citizens. She will have to attend her visa interview first and then go through the regular I-601 process in order to receive a visa to reenter the United States.

You cannot apply for the I-601A waiver until the I-130 petition is approved, transferred to the National Visa Center and you pay the IV fees.