Natural Hypnosis refers to hypnosis that occurs naturally without the inducement of a hypnotist. Eg when you are absorbed while reading a favourite book.
How frequent does it occur? The answer is very often. But the frequency also depends on individual and surrounding factors.
If a person does not sleep well or is in constant fatigue, then he may enter into hypnosis easily. It also does not matter the time of the day, for hypnosis can happen at any time.