In plain words
When your buddhi crosses beyond the thicket of delusion, you will become indifferent to what has been heard and what is yet to be heard.
Word for word
When your buddhi crosses beyond the thicket of delusion, then you will come to indifference toward what is to be heard and what has been heard.
The commentators
Bhagavadgītābhāṣyam
योगानुष्ठानजनितसत्त्वशुद्धिजा बुद्धिः कदा प्राप्स्यते इत्युच्यते —
यदा यस्मिन्काले ते तव मोहकलिलं मोहात्मकमविवेकरूपं कालुष्यं येन आत्मानात्मविवेकबोधं कलुषीकृत्य विषयं प्रत्यन्तःकरणं प्रवर्तते, तत् तव बुद्धिः व्यतितरिष्यति व्यतिक्रमिष्यति, अतिशुद्धभावमापत्स्यते इत्यर्थः । तदा तस्मिन् काले गन्तासि प्राप्स्यसि निर्वेदं वैराग्यं श्रोतव्यस्य श्रुतस्य च, तदा श्रोतव्यं श्रुतं च ते निष्फलं प्रतिभातीत्यभिप्रायः ॥ ५२ ॥
In plain words
When does that understanding arrive — the understanding born of the clear mind that the practice of yoga produces? To this, it is said —
'When' — at whatever moment — the thick tangle of delusion, this murkiness that clouds your clear sense of what is Self and what is not, and pulls your mind out toward sense-objects, is finally crossed by your own understanding, your understanding will settle into a state of real purity. That is the meaning. And at that same time, you will arrive at dispassion — toward both what you have yet to study and what you have already studied. Both will simply seem to hold nothing more for you. That is the point being made here.
Close to the Sanskrit
When will the intellect that is born of the purity of mind produced by the practice of yoga be attained? Thus it is said —
'When' — at whatever time — the thicket of delusion, of the nature of delusion, in the form of non-discrimination, [that is,] the murkiness — by which the mind, having muddied the discerning cognition of Self and non-Self, proceeds toward [sense-]objects — then your understanding will cross over, will pass beyond [that murkiness]; it will attain a state of exceeding purity: such is the meaning. Then, at that time, you will go to, you will attain, dispassion toward that which is to be heard and that which has been heard; then both what is yet to be studied and what has already been studied will appear fruitless to you: such is the intent. [End of commentary on verse 52.]