Srimad Bhagavatam Canto 06, Chapter 12, Text 33

SB 6.12.33

vajras tu tat-kandharam asu-vegah
 krntan samantat parivartamanah
nyapatayat tavad ahar-ganena
 yo jyotisam ayane vartra-hatye
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Although the thunderbolt revolved around Vrtrasura’s neck with great speed, separating his head from his body took one complete year — 360 days, the time in which the sun, moon and other luminaries complete a northern and southern journey. Then, at the suitable time for Vrtrasura to be killed, his head fell to the ground.
Srimad Bhagavatam Canto 06, Chapter 12, Text 32
Srimad Bhagavatam Canto 06, Chapter 12, Text 34