Hi Parag! After offboarding around 8 pm, you would be at a near-perfect timing to catch a Volvo bus from ISBT/Mandi House/Majnu ka Tila towards Manali. On an average, buses to Manali drive every few hours from Delhi till 10-11 pm at night.
There are so many Volvo Buses Running from Delhi to Manali in Evening. So you can take bus from Majnu ka Tila, New Delhi and Reach there. Hope You will get the proper answer regarding your question. Happy Journey..!