Verse 18

ततो वर्षसहस्रेण तपसा दारुणात्मना ।प्रापदभ्यासवशतो ज्ञानमात्मप्रसादजम्

In plain words

Then, through a thousand years of tapas fierce in its severity, by the strength of steady practice, he attained the jñāna born of the Self's own grace.