Yogeshwarananda explained that in some cases, a meditation which achieves a blank state, with a few ideas arising now and again, should continue for some time, for at least fifteen minutes. If perchance the yogi can do pranayama practice before the meditation, then it is likely that he/she will stay in the blank state for a longer period and with little or no effort to remain there.
One should learn to wait for developments when in that state. The question of how long arises. The answer is that, if necessary, one should be prepared to wait forever. In the Puranas, we read of ascetics or even lower deities, who went to an edge of reality, and waited or called for a superior personality to speak or appear. The wait for this in some cases was thousands of years, a period of an era, a long time.
The demigod Brahma may wait for several eras of time, for his guru, Lord Vishnu, to appear. Or Brahma may only hear a word or two spoken by Vishnu. He may not see the deity.
If during meditation, the yoga guru appears in the mindspace of the yogi, that is wonderful but one should not wait in anticipation that this will happen. One does not know how the communication will be expressed. If one has an expectation that may cause stress during the meditation which will prevent one from reaching higher planes.
During a meditation session, when Yogeshwaranand entered my subtle head, he instructed that I gather the focusing energy which was randomly sprinkled in the mind. This was like gathering dust which floated on water. When this is done, a yogi should focus on the collected energy. He should keep the attention there. This may cause one to gain access to chit akash sky of consciousness or to a supernatural plane of existence.