The amount of time it takes to get used to bifocals depends on each individual. When people wear a ring or watch, they don't feel it at all after a while. Getting used to bifocals is the same concept.
Many people find that if they move their eyes more than their heads when reading, they get used to bifocals faster. Because there is a foreign object on the face, some people may notice tension in their neck or back as well as additional headaches at first. These aches and pains occur because of the up-and-down movement of the neck when reading.