Different medicines work on very different timescales. Fast-acting medicines like many painkillers, antihistamines and rescue inhalers are designed to relieve symptoms within minutes to an hour. Antibiotics often start fighting infection quickly but may take a day or two before you feel noticeably better.
Other medicines are meant to build up gradually or to control a condition over time rather than give instant relief. Antidepressants, many blood pressure medicines, thyroid medicines and some arthritis treatments can take several weeks to reach their full benefit, and you may not feel any obvious change early on. This does not mean the medicine is not working.
Knowing the expected timeframe helps you avoid stopping too early or assuming a medicine has failed. If you have passed the expected window and see no benefit, or your symptoms are getting worse, contact your doctor or pharmacist rather than increasing the dose on your own.