The glasses are worn during the day for clear vision for both objects far away and nearby. Generally they are worn every day although occasionally a day without the lenses will not adversely affect the treatment. Objects in the distance are viewed in the upper portions of the lenses and objects nearby are viewed through the lower portions. The lenses are worn as long as myopia progression is considered a risk.
The short story is that they don't work very well. Bifocal and PAL (Progressive Addition Lens) glasses have been shown in published works listed in the Glasses section of the Research menu to slow the progression of myopia slightly. The effect of wearing the lenses was a 67% progression rate the first year, but no reduction in rate for succeeding years (100% progression).
We are not sure why bifocal type lenses work somewhat, but it may have to do with creating an area of myopic focus on a portion of the retina, thought to slow myopic progression. You can read about peripheral focus in the different sections of the Definitions menu. Here's a link to the hyperopic defocus section. The fact that bifocals, along with other treatments, stop working after about a year is also being studied.
PALs have been shown by Gwiazda (2004) to most benefit a subset of patients who have a near esophoria and a lag of accommodation greater than .50D, something your optometrist will have to determine. If you are in that smaller group, PALs were shown to have a progression rate of 56% over three years, much better than the group as a whole.
Children can wear bifocals or PALs easily. They usually adapt well. Their potential benefit is reduced in that they commonly slip out of position so that the focus changes being created are out of position also.
Bifocal and PAL (Progressive Addition Lenses) are prescription lenses for spectacle frames worn on the face to change the eye's focus. If the eye is nearsighted (shortsighted or myopic) for example, a lens is normally designed to give clear distance vision. Sometimes this is called a monofocal (one-focus) lens. A bifocal (bi or two-focus) lens has two powers which can be located at different locations on the lens. In glasses, the bifocal power is usually (not always) located at the bottom of the lens. PALs have multiple reading powers below the central vision to allow vision at different distances without the eye's focusing ability being necessary.
Bifocals and PALs are normally used for the condition of presbyopia, which is what happens to everyone, usually in their forties, when it becomes difficult to focus on both distant objects such as street signs and near objects such as a book. The solution is to make the corrective lens have two powers, one for each distance. Many types of bifocal, multi-focal and progressive power glasses have been designed for various occupations and tasks. The eye turns behind the lens in the frame to view through different portions of the lens to obtain the power necessary for the object being viewed. A common design would have the person looking straight ahead to focus on distant objects and looking down to focus on near objects.