How long you fast for will depend entirely on your motivation for doing so, as you have said, and where this is lacking you will need to discover reasons for yourself.
Fasting for weightloss might not be a strong enough reason in itself, but with some people who have had a serious disease, this would be a powerful motivating force in itself and very little encouragement would really be necessary.
Would you say you have a food addiction? because this can be a problem in fasting as with any other addiction.
For whatever reasons you are trying to fast, I suggest having a few books on fasting available to read both before and during the fast, which can act as an inspiration and motivator; otherwise the company of fellow fasters in a specialized retreat can easily provide the right environment & encouragement instead of trying to go it alone, with only this forum for inspiration. Many members though have found this forum to be a very strong motivator to actually go the distance, such as Mouseclick, Bella Lucia, and L*u*l*u to mention but a few.
Also, bear in mind that it is usually the first week that can be the most severe with detox symptoms but as the fast progresses this can easily ease into a more pleasurable experience. It can be just a case of mind over matter and the willpower to stay the distance in the short-term, for the longer-term benefits to manifest themselves: it really is well worth the effort, I can assure you.