SB 6.12.33

vajras tu tat-kandharam āśu-vegaḥ
 kṛntan samantāt parivartamānaḥ
nyapātayat tāvad ahar-gaṇena
 yo jyotiṣām ayane vārtra-hatye
Word for word: 
vajraḥ — the thunderbolt; tu — but; tat-kandharam — his neck; āśu-vegaḥ — although very fast; kṛntan — cutting; samantāt — all around; parivartamānaḥ — revolving; nyapātayat — caused to fall; tāvat — so many; ahaḥ-gaṇena — by days; yaḥ — which; jyotiṣām — of the luminaries like the sun and moon; ayane — in moving to both sides of the equator; vārtra-hatye — at the time suitable for killing Vṛtrāsura.
Translation: 
Although the thunderbolt revolved around Vṛtrāsura’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 Vṛtrāsura to be killed, his head fell to the ground.