You don't have to shave all your existing hair off. In fact leaving a bit at the back and sides can enhance the realism of the system at its extremities.

As for longevity, it depends on how you treat it and whether or not you sleep with it on. Count on an average of 12 to 14 weeks; I've got 5 months out of one full cap, others have managed longer. Drying out of the hair and shedding are the two main issues, fraying can also reduce useability. The type of base may also determine how long it lasts - generally injected skin bases have a shorter life span than the knotted lace versions.

Full caps cost more and can take longer to make. They can also cost more to get cut in. But they can solve most of your hair issues in one hit if you get the style and density right making them a very attractive total solution.