I often get inquiries on how long records should be kept. My normal answer is to keep tax returns, general ledgers, & financial statements permanently and supporting documents such as cancelled checks, paid invoices, billings, check registers, etc. for 7 years. Remember supporting documents on purchases of real estates, stocks, bonds, etc. should be kept for 7 years beyond the date the underlying assets are sold.
Here is link which includes additional information. While that site recommends a shorter period of some records, there may be other reasons for keeping some records a bit longer.
The IRS also has a page on recommended record retention, http://www.irs.gov/businesses/small/article/0,,id=98513,00.html. But my recommendation is to use the IRS recommendation as a reference only.