You need to think of a number of things, which can affect the exposure time. They include the type of enlarger, the size of the enlarger as well as the iris, both control the amount of light reaching the lens and hence the exposure time. In addition, the chemicals play an important role in the process, but these are non-modifiable factors, which you cannot influence.
You also need to take care of the paper and the time that the picture is in the developer to make sure that the exposure time is perfect for creating the perfect image.