What date can I look at to determine when bad debt will fall off my credit report?

Typically, 7 years from the date of the last activity on the account.

So, if an account is showing last activity over 7 years ago, I can dispute and have it removed?

Yes you may contact that reporting agency ( may as well check the major three and do them all at the same time)

Some things like bankruptcy will normally show up for 10 years.

Online info for each, including how to dispute:

Trans Union (http://www.transunion.com/index.jsp)
Equifax (http://www.equifax.com/)
Experian (http://www.experian.com/)