nooluv,
You should not activate it more than once every couple of hours. The average use should be around once a month.
Here is a quote from a Glossary:
Ever wonder what that "degauss" button on your monitor does besides make a buzzing noise and cause the screen to go crazy for a second? Though that's its main purpose, the degauss button has another useful feature. To understand it, you'll first need to know that the earth has natural magnetic fields. The magnetic charges from these fields can build up inside your monitor, causing a loss of color accuracy. Degaussing scares the bad magnetism out of the monitor and fills it with good karma. If your monitor doesn't have a degauss button, fear not -- many new monitors automatically degauss themselves. If you have a flat-panel display, there is no degauss button because magnetism doesn't build up in flat screen displays.
Take Care,
Richard
P.S. There is a little humor in the definition above. To degauss is to demagnetize. Degaussing a computer display removes any undesirable magnetic field that may have built up within the display, causing distortion of the image or color. Display monitors with cathode ray tube (CRT) technology are subject to the buildup of magnetic fields. Many display manufacturers include an internal coil that will degauss the display when it is turned on. Some displays also include an external control that a user can operate to degauss the display at any time.