When is damaged US twenty dollar bill no longer any good?

A local banking representative would be able to relay such info to you - the decision to maintain a specific $20 bill in circulation is based on its condition (Is there writing all over? Is it half torn?, etc). If you have a damaged bill, you can exchange it at a bank as long as you have more than half of the bill. As bills come into a bank, they decide which ones are too worn and limp to continue circulating and return them to the Federal Reserve Bank where they are destroyed. The FRB replaces them with new bills.