How Long Does Erection Of Penis Last Before Ejaculation?

sir, i'm ravi ,a 19 year old guy and .i'm intrstd in mastrbatng. my penis will erect upto 14cm.but there is no ejaculation of sperm. but sometimes i used to have some sexy dreams when i'm sleeping. but at that time there will be ejaculation. can u clearly say the basic methods of masturbating and exactly the time it will take sperm to ejaculate? and i dont have tolerance to wait for the sperm to ejaculate.