Changing electric guitar strings is not that hard and something you should become comfortable doing on your own. You should change your strings on a regular basis. For example, if you start to see an oily, dirty build up on your strings then you should probably change your strings. The dirt from your hands and fingers is collected by the strings and will kill your tone.
Active players or professional players may change their strings one a week or more, especially if they are playing in public. Some players change strings just before each show.
A Word About String Gauges
Strings come in variety of gauges. The gauge of the string tells you the thickness of the strings. The lower the gauge the thinner the strings, higher gauges mean thicker strings. Most manufactures display the gauge size by the size (diameter) of the high E string (the smallest string). Typical gauges run from .008 to .012.