There is only one bus that goes to kaza from kinnaur. it starts from shimla at 6 i the evening but is usually packed. So make sure you reach there a couple of hours early and get youself a ticket with a seat number because otherwise you may have to go standing all the way to kaza, the bus will reach the reckong peo bustand in the middle of the night, around 3 in the morning and you will spend around 3 to 4 hours there until the connecting bus pulls in around 7 a.m. which will take you to kaza. the best thing about getting a seat number from shimla is that you`ll get a priority over seats, the people who get on from reckong will probably stand in the bus all the way to kaza. and you should reach kaza around 5 next day. It`s a long journey and a little hard on the ass , but the views from reckong to kaza are amazing you`ll love it. I say, it`s totally worth it!