Eire,
The length of both the work and rest intervals should be determined by your goals/what (if any) activity you are working for. If you have a specific goal (eg to prepare for sport), then one should perform an activity-analysis to determine what would be the dominant energy systems utilised (ie the ATP-PC, Lactic Acid, or Oxidative system).
By knowing what your development of these systems is compared to what is desirable, one can then manipulate the work and rest intervals (and intensity of each) in order to improve the specificity of your adaptations.

If your goals are for health or fat loss, then we can possibly be a little more lax in our prescription of intervals.

If you want to share your goals I would be happy to give some suggestions, however, some general principles:
- the shorter the rest periods, the more 'aerobic' the session becomes
- the higher the intensity, the higher the EPOC is (possibly leading to greater fat loss).
- the length of the work interval and it's intensity will be inversely proportional

A large number of article on interval training can be found at Peak Performance. Brian Mac's pages offer an easy to understand array of exercise physiology articles, like this one on lactate threshold - an important physiological parameter to enhance for athletes that require cardiorespiratory fitness.

If you have specific questions about manipulating the parameters of an interval session I would be happy to try and answer.

Kevin