I never recommend relying on lactation amenorrhea (no period) as a form of birth control.
Menstruation is most likely to return with prolonged periods of physical separation from the baby and/or longer than typical intervals between breast stimulations.
You need to know a few things about your menstrual cycle returning:
In some lactating parents, ovulation and/or menstruation may temporarily decrease your milk supply, most often rebounding as hormone levels normalize. If you notice a change it supply, it will likely be right around when you are ovulating. You may notice an increase in nipple tenderness and a decrease in pumping output/ frustration or increased frequency of nursing. Your period probably won’t be regular when you first get it back, so you may not know when you ovulate.
A Calcium Magnesium supplement can stabilize supply throughout your entire monthly cycle. Take a maintenance dose of 500 calcium/250 magnesium all month long and increase it to 1500 calcium/750 magnesium ovulation through 3rd day of your period.
Certainly, a return in your period is a return in the need of fertility. If you aren’t already, protect yourself from pregnancy with your chosen form of birth control.
Birth control options: Non-hormonal or progesterone only option. Do not take any estrogen containing birth control, as estrogen is known to significantly decrease milk supply.