The typical estrus cycle, or heat cycle, in cats lasts about 2 to 3 weeks and can recur every few weeks or months if the queen does not get pregnant. During the estrus phase, queens are receptive to males and display various signs of being in heat, including vocalization (calling or yowling), increased affection, rubbing against people and objects, rolling around, and lifting the hindquarters when approached by a male.
Since you mentioned that your queen had a cycle when her kittens were two weeks old, it is likely that she went into heat shortly after giving birth. This is not uncommon in cats, and it's known as the "postpartum estrus." It's important to be aware of this possibility, especially if you have male cats or want to avoid her having another litter too soon.
To prevent unplanned pregnancy, it is generally recommended that female cats be spayed (neutered) before they reach sexual maturity, which is around 6 months of age. Spaying involves the surgical removal of the ovaries and uterus, which stops the heat cycles and eliminates the risk of getting pregnant.
By keeping your queen indoors during her heat cycle, you can help reduce the chance of her mating and becoming pregnant. However, it's worth noting that queens can still go into heat even while taking care of their kittens, and some males can be persistent and find ways to get to them. Therefore, spaying is still the most effective way to prevent unwanted litter and potential health risks associated with repeated pregnancy and giving birth.