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SB 10.3.36

evaṁ vāṁ tapyatos tīvraṁ
 tapaḥ parama-duṣkaram
 dvādaśeyur mad-ātmanoḥ
evam — in this way; vām — for both of you; tapyatoḥ — executing austerities; tīvram — very severe; tapaḥ — austerity; parama-duṣkaram — extremely difficult to execute; divya-varṣa — celestial years, or years counted according to the higher planetary system; sahasrāṇi — thousand; dvādaśa — twelve; īyuḥ — passed; mat-ātmanoḥ — simply engaged in consciousness of Me.
Thus you spent twelve thousand celestial years performing difficult activities of tapasya in consciousness of Me [Kṛṣṇa consciousness].