Some salons will still do highlights by putting a plastic cap on over your hair, and pull sections of hair through the cap in places. Then apply the highlighted color to these sections, so the color does not go through the cap to the other hair. This can be less expensive but most are doing the foiling now as described above, painting the color on with a brush, wrapping in foil, and sitting under the heated dryer for a bit.

Either process can range from $35-$80 depending on your area and the salon you go to. Call around to price it and ask which method they use. The disadvantage of the cap is that it can be uncomfortable, especially if your scalp is sensitive, as the hair is actually being pulled through and can tug at the scalp, and the tool they use to pull can be annoying if they are not experienced, but is less expensive. Foiling is more comfortable but usually a bit pricier, not by much though.

Highlights will last as long as your hair is. New hair growth, which is about a 1/4 of an inch a month, roots will show in anywhere from 4-6 weeks depending on growth and the color chosen.

As an example, the highlights I have now are not far off from my base color, very subtle so the growth is not as noticeable and I can get them redone every 4-5 months without there be much notice. It may show more though depending on how much difference in color you want, needing to be done more often if the color difference is more significant. Some do it religiously every 4 weeks, some about 7 mos. Depends on the color, price, and how you like your roots.