SB 10.3.36
evam vam tapyatos tivram
tapah parama-duskaram
divya-varsa-sahasrani
dvadaseyur mad-atmanoh
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Thus you spent twelve thousand celestial years performing difficult activities of tapasya in consciousness of Me [Krsna consciousness].