How Long Does Caffeine Stay in the Body? In most cases, caffeine has a half-life of about 5 hours (meaning 5 hours is needed to eliminate half the coffee amount). However, it might take longer – for instance, a pregnant woman might have caffeine in her system up to 20 hours after she ingests it.etween 150 and 200 milligrams is considered safe in most studies, but many doctors recommend pregnant women avoid caffeine altogether. That’s because no matter how long caffeine stays in the body of the mother, it might stay even longer in the baby.