How often does a healthy young woman need a check-up?

If you are healthy, I still think a once yearly check up is optimal for most women. It is healthy to maintain a relationship with your doctor. This way anticipatory health guidance and future preventative care can be discussed. It is always better to be proactive about your health than wait until you have problems to develop. A once-yearly visit even while "healthy" can take the fear out of going to the doctor. Not a lot of testing needs to be done at the visit, but even small changes in one's health can be important to keep on top of. 

There are varying recommendations. The American College of Obstetricians and Gynecologists recommends that women get an annual physical exam. At a minimum, it is recommended that all women ages 21-65 obtain a pap smear every 3 years starting at age 21. Women younger than 21 do not need a pap smear.