Though the reasons for this are not known, geological evidence indicates that the strength of the field gradually grows weaker, reaches a minimum or disappears entirely, and then builds again the opposite direction, resulting in a reversal of the north and south magnetic poles.
Scientists estimate that such reversals occur about every one-half to one million years.
The most recent reversal took place about 700,000 years ago. Calculations are that if this degradation continues at its present rate, there will not be a sufficient magnetic field to support life within 1500 years. This amazing site has a series of brief, but powerful articles about magnetism.