Yes, the ideal of a Lhasa is to have a long hair, but it takes time and not all Lhasas have the same long hair. Which means that you shouldn't worry about it.
Yes, for my dog it takes about 6 months for long hair. If you want hair that touches the floor it should take about 2 year. But remember to groom it every day because it gets mats easily and you don't want that.