How far back can they make us go in searching records for a subpoena? Is there a limit on the number of years? Where can I find this information. Thanks for any help.

State law and retention requirements would be a start. Also, if you don't destroy according to a set retention period, they can reach anything still in your possession.

We actually only keep closed accounts for five years, so if it is closed, five years, but what about history on an open account?

See above. If you have them, they are discoverable.