Srimad Bhagavatam Canto 10, Chapter 03, Text 36

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].
Srimad Bhagavatam Canto 10, Chapter 03, Text 34-35
Srimad Bhagavatam Canto 10, Chapter 03, Text 37-38