First let me say, that there is No Such Thing as a Teacup, Micro, Mini or Pocket Size Chihuahua. Those are just terms, that irresponsible breeders use, when they breed one Runt (smallest puppy in the litter) to another Runt, in order to achieve much smaller puppies. Those so-called breeders don't care anything about the welfare of these poor dogs or if they live or die, trying to deliver these puppies. If a Runt dies, they just move on to the next Runt and the cycle just repeats itself! All they care about is the money and charging people insane prices for these puppies! Those are the people you need to stay far away from. A Runt, should Never under any circumstances be bred. Their bodies are not built to carry and deliver puppies. They are too tiny and it's way too dangerous to even consider breeding them. In answer to your question, they stay in heat for 3 weeks. One week going in, one week in and one week coming out. Please keep your female away from all males that are not Neutered. When she goes outside to go potty, you have to go with her and stay with her until she finishes, then bring her back inside. Never, ever leave her alone outside or around a male that is not Neutered! A male can smell a female in heat from miles away and will do whatever he has to, to get to her, breed with her and get her pregnant! If you have a male that is not Neutered then you need to separate them both and keep them apart for about 3 1/2 weeks to be safe. Your male will whine, cry and want to get to her, but you cannot let him. He will also lose interest in eating much until the female comes out of heat and he will lose weight. I used to breed Chihuahuas! I now have had all my males Neutered and my females Spayed. If you have any questions, give me a shout and I will do my best to answer them. Please consider having your female Spayed after the vet says it's safe!