Premature ejaculation is the most common sexual dysfunction in men under 40. Most therapists who treat premature ejaculation define it as ejaculation happening before both sexual partners want it. This avoids specifying an exact length of sexual intercourse, which depends on many things specific to the individuals concerned.
An occasional experience of premature ejaculation might not be cause for concern, but when it happens more than 50% of the time, treatment may be appropriate.
So, while a man may reach orgasm after (say) 8 minutes of sex, this is not premature ejaculation if his partner comes within 5 minutes and both are happy about it. (Nor, indeed if his partner does not come at all. Very few women do during sexual intercourse.) But another man may last 20 minutes before he comes, yet he may think this is premature if his sexual partner needs 35 minutes of stimulation before reaching orgasm.
Many women are unable to orgasm through vaginal intercourse no matter how long it lasts, so this highlights the importance of being flexible about both the definitions of PE and the premature ejaculation causes.
Premature ejaculation has 3 characteristics as defined in the Diagnostic and Statistical Manual (DSM-IV-TR) as (1) persistent and recurrent ejaculation with little sexual stimulation, around the time of penetration and before the person wishes it; (2) distress or interpersonal issues; and (3) not exclusively due to some other cause.
Premature ejaculation is defined as primary or secondary. Primary premature ejaculation means you have ejaculated too soon since your first sexual experience. Secondary premature ejaculation begins later in life, after previously successful sex, and the cause is unknown.
Premature ejaculation fits best into the category of “not otherwise specified” because the cause is unknown, although psychological factors are suggested in most cases.