Fusing glass is basically stacking layers of glass, 2 or more, and heating (firing) them in a kiln (a special oven for glass) in very high temperatures in order to unite the sheets of glass together.
The length of time in which the kiln will operate determines the final results in bonding the pieces of glass. Since glass is a very sensitive material it is essential to be very precise in deciding how high the temperature will be and for how long the kiln will operate in that specific temperature. The whole process of glass fusing may take around 20 hours until the glass is properly chilled and you are able to open the kiln and see the flat and finished product.
If you want to prepare a serving dish for example, that would require an additional process called slumping, which means using a mold in order to create a shape out of the flat glass. In this process relatively lower temperatures are applied which shortens the time by a few hours.
Should you have any question, please do not hesitate to contact me.