How long is the impersonal conception useful?

The impersonal conception of the Supreme Absolute Truth, as described in this [twelfth] chapter, is recommended only up to the time one surrenders himself for self-realization. In other words, as long as one does not have the chance to associate with a pure devotee, the impersonal conception may be beneficial. In the impersonal conception of the Absolute Truth one works without fruitive result, meditates and cultivates knowledge to understand spirit and matter. This is necessary as long as one is not in the association of a pure devotee. Fortunately, if one develops directly a desire to engage in Kṛṣṇa consciousness in pure devotional service, he does not need to undergo step-by-step improvements in spiritual realization.

Bhagavad Gita As It Is 12.20 purport