Binaural beats work in different frequencies to provide the listener with different results, therefore as a person who practices the binaural beats 8 Hz therapy you will be looking for the result provided by the Alpha waves range.
The Alpha waves range include beats between 7 and 13 Hz, but most people benefit from the binaural beats 8 Hz for relaxation, mediation and being able to focus on specific tasks. As each individual is different, the affects you experience from binaural beats 8 Hz therapy might not be identical to that of your friend.
People who have experienced that binaural beat therapy 8 Hz helps them to feel relaxed will find that even short sessions during the day will provide the same positive benefit, while longer sessions at night before bed will allow them to have a restful sleep.
Some people will also find that only certain frequencies in the Alpha range actually give them any benefit, this is also because individuals are different and therefore the brain will react differently to various frequencies.
Individuals with epilepsy should never consider binaural beats therapy as it has been known to cause adverse effects.