Lhasa Apsos normally get their first period or enter into their heat cycle at the age of 6-months. They usually go into heat twice a year. You should get your dog checked by a vet if she seems to be in heat every month.
How Many Times Do The Lhasa Apsos Get Their Period And How Long Does It Last ?
Normally its every 6 months
To add to the other answer, the estrus cycle lasts around three weeks.