How long do you have to take Macrobid to prevent uti's?

I have seen it used long term in some patients (maybe not forever but long term). The length of treatment would depend on your recurring symptoms and would depend on your doctor

Dosing

The dose of this medicine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of this medicine. If your dose is different, do not change it unless your doctor tells you to do so.

The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.

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* For oral dosage forms (capsules, suspension, and tablets):
o For prevention of urinary tract infections:
+ Adults and teenagers—50 to 100 milligrams (mg) at bedtime.
+ Children and infants 1 month of age and older—Dose is based on body weight and must be determined by your doctor.
+ Infants younger than 1 month of age—Use is not recommended.
o For treatment of urinary tract infections:
+ Adults and teenagers—50 to 100 milligrams (mg) every six hours.
+ Children and infants 1 month of age and older—Dose is based on body weight and must be determined by your doctor.
+ Infants younger than 1 month of age—Use is not recommended.
* For oral dosage form (extended-release capsules):
o For treatment of urinary tract infections:
+ Adults, teenagers, and children 12 years of age and older—100 milligrams (mg) every twelve hours for seven days.
+ Children younger than 12 years of age—Use and dose must be determined by your doctor.

http://www.drugs.com/cons/furadantin.html.