How many days does a dog stay in heat?

My Lhassa Apso Tinsel was on heat for 17 days....and she mated with my male on the 17th day.....so hopefully we will have a litter from her for the first time.i was,nt really bothered about breeding her as this is her first time....my other Lhassa Hollie had a litter in Janruary 2011 they were a beautifull litter of four puppies....i did sell three of them .i do keep in touch with the buyers.i am very fussy who i sell my puppies too and i cried when each one of the puppies left.i only caged my puppies at night for safety.i loved having them running around and they was such happy puppies.again hollie mated on the 15th day only and had four puppies......regards Wyn