Depending on your height and weight, an antibiotic's half-life may help determine how long it will stay in your body. Each antibiotic has a different half-life and can range from short half-lives of only an hour to half-lives over 68 hours before half of the drug has been eliminated from the body. The short answer is that it takes 5 half-lives ( read more about half-lives and how a drug gets out of the body). Each drug has a different half-life. Here are the figures for sleep meds: Sleep Med. Half-life. Melatonin. 40-60 min. Ramelteon (Rozerem) 1-2 hr. The fundamental approach begins with activity, as @Declan suggested. A billion years is still quite fast as you then only need a small amount of the substance to get a large number of decays per second. It becomes a lot more difficult for half lives longer than 1018 10 18 years, see e.g. here. A medication's half-life is when it takes about half of the medication to leave the body. It takes about five half-lives for a medication dose to leave the body. The half-life of both forms of hydroxyzine ranges from 14 to 25 hours. Thus, the medication can remain in the body for about 70 to 125 hours (five half-lives) after the last dose. 1. Add a comment. -2. Half life wiki speculates that most likely setting of Half life 1 is in May 2003. Portal 1 sets around 2010. Maybe year or two after 7 hour war. Half life 2 and episodes happens around 20 years after first one. That's around 2023. I like to think these as official setting years. "There is, however, some evidence from the Half-Life 2 Episode 1 official website indicating that Half-Life 2 takes place nearly two decades after the Black Mesa incident." Reply reply ultimate_fatass0921 In the average adult, the half-life of caffeine is about 5-6 hours. This means that once take a dose of caffeine, you'll break down about half of that caffeine after 5-6 hours. So if you take in 200 mg of caffeine at 9 am, you'll still have about 100 mg left in your body between 2 and 3 pm. XzuH.