How long should inquiry be practised?

As long as there are impressions of objects in the mind, so long the inquiry ‘Who am I?’ is required. As thoughts arise they should be destroyed then and there in the very place of their origin, though inquiry. If one resorts to contemplation of the Self unintermittently, until the Self is gained that alone would do. As long as there are enemies within the fortress, they will continue to sally forth; if they are destoryed as they emerge, the fortress will fall into our hands.

What is the nature of the Self?