When I feel orgasm approaching and ejaculation is imminent. Then I pull out and do it on her stomach. This works best as a secondary method. Don't trust it as your primary method, too risky.
How Do Boys Know When To Pull Out In Time?
There is no safe time, as the lubricant that boys release during the act and even upon penetration will contain sperm so this is far from a safe way of preventing a pregnancy. Other methods like a condom with a spermicide would be safer, as well, if going on the pill, must wait at least one cycle before it is effective but does not protect from std's
They don't know they just tell the girl that so she wont be as scared about getting pregnant