One verse

Srimad Bhagavatam 10.3.36

10.3.36

एवं वां तप्यतोस्तीव्रं तप: परमदुष्करम् । दिव्यवर्षसहस्राणि द्वादशेयुर्मदात्मनो: ॥ ३६ ॥

evaṁ vāṁ tapyatos tīvraṁ tapaḥ parama-duṣkaram divya-varṣa-sahasrāṇi 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].