Albeit obscure, this was the only information I was able to find on your issue,
Manali-Kaza: When this 210 km route is open, there are two daily ordinary buses leaving from Manali Bus Stand to Kaza at 5.00 AM and 5.30 AM. These buses are run by Himachal Roadways and fare is approximately INR 200. The bus passes through Rohtang Pass, Batal, Kunzum Pass, Lossur and takes about 11 hours to reach Kaza.
Important point is that once the route is declared open by the authorities, you do not get the public buses for next 2-3 weeks. They only allow small vehicles to run on the road initially. In this period, you can get a shared sumo/jeep. A shared sumo between Manali and Kaza cost you Rs.700 and takes 9 hours to complete this trip.
I'm not going to say that you should take mine or this man's word for it, but it would be similar to taking someone else's word on it. Most others will be able to tell you something similar by experience as it seems that published information regarding routes in this area is rare if any.