Dollar bills are actually made from more than just wood pulp, they also include cotton, silk and linen -- along with red/blue fibers inserted to create texture.

According to the federal reserve, bills have the following lifespan before they wear out:

$ 1.....22 months

$ 5.....16 months

$ 10.....18 months

$ 20.....24 months

$ 50.....55 months

$ 100.....89 months

http://www.federalreserve.gov/generalinfo/faq/faqcur.htm#13