This is a nice break from making the juice yourself. I've even known people to get great results from drinking store-bought bottled juices, so please don't use the excuse that you don't have time to do a juice fast. With a little planning, everything should happen without a hitch. A good general rule of thumb is to fast one day a week. Another option is to fast for 3-5 days per month.
Some people do long fasts - sometimes up to a month of juice fasting, once a year.
These options are all good for maintenance and for some extra housecleaning of the body. For more serious or life-threatening situations, more fasting may be indicated. For a cold or flu, the first thing to do is fast. Juice fast until it's gone.