It depends on the transistor being used. All transistors are not created equal and they are not the same. There is no way to answer your question which is what I was getting at. The link tells you how in a basic application to calculate base resistors. You can't just toss out "a transistor" and expect a simple answer when there are thousands or transistor devices and each has a data sheet. If you are looking at a drawing using for example a 2N2222 basic switching NPN transistor then you would pull up a data sheet for that transistor.
Typically the base-emitter voltage drop of a silicon bipolar transistor is one diode-drop or about 0.65V. You subtract that from the base resistor to emitter voltage to get the voltage across the base resistor. The base current is then the resistor voltage divided by the base resistor value.