Does precum contain enough sperm to cause pregnancy? how long does precum occur before ejaculation?

Usually sperm count in pre-ejaculate fluid is very low, insufficient to allow conception. Once in a while it can happen, but quite rarely. Pre-ejaculate fluid is produced as soon a the penis becomes erect. To be maximally safe, there should be NO penile penetration, even if ejaculation is planned outside the vagina. If you are sexually active but avoiding pregnancy, use proper contraception.

Precum is the secretion from the glands lining the passageway at the time of sexual excitement well before ejaculation and by itself does not have any sperms. The time depends on the duration of excitement and time of ejaculation which varies from one episode to another.