Setting time of the concrete depends on the rate of hydration process (chemical reaction between cement and water).
Initial setting time of the concrete could be near to same of the cement used for producing it. As per codal provision it should at least provide us 30 minutes of time after adding water in it to work with. i.e. - initial setting time = minimum 30 minutes.
This initial setting gets delayed further due to additional of chemical admixture such as retarder, super plasticizers etc. they keep water away from cement for the time for which there chemical reactions chains are designed.
After initial setting, it moves to final setting and get converted into harden set.