there really is no specific time weather in minutes or in seconds for you to consider your self a premature ejaculator.
the concept of premature ejaculation is that you have a rapid ejaculation, rapid climax, premature climax or early ejaculation. so if you think you ejaculate earlier than normal so you can consider your self a premature ejaculator.
Duration of ejaculation is completely related with how horny you are!
so if you are very horny you will hae an early ejaculation and if you are less horny you will hae late ejaculation!
Try to do the stop and go method. You could last a long time.