Ones that are faster acting have shorter half lives (a half live is the amount of time it takes for half of the steroid to decay).
What that means in practical terms, is that steroids with shorter half lives need to be administered more frequently than those with longer half lives, in order to keep your blood levels stable.
For example, dianabol has a half life of about 4 hours. So, most people take a dose every 4 hours, in order to keep even levels of the drug in their blood.