All medicines are different. Some are almost instant- they may take only a few minutes to work. Others are designed to release only once they enter your intestine (could be hours later). Still others are designed to release the drug slowly over time.
Some medicines are affected by whether you take it with food or have recently eaten. Acid in the stomach and transit to the intestine as well as the liver's capacity to metabolize the medicine could affect the drug as well.
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The absorption rate depends entirely upon the medication and the individual person. Some medication have larger molecules and also they are designed to "break down" slower in the body, therefore, they would be absorbed slower. Also, a person with a disorder of the gastrointestinal tract will also have a problem with absorption. Peptic ulcers, gastroenteritis, many other things can change absorption rates of medications. This question would be difficult to answer as a general question.