What log files should we be keeping on our interface engine and for how long?

First, let me clarify that I am only going to refer to interface engine related logs and not window's or server logs.  Obviously, servers have a finite amount of space, so you need to use it wisely; otherwise, you run the risk of crashing the server.  When I am setting up an engine, here is what I think works well. 

For the inbound interfaces, I would recommend that you save only the pre-process transactions.  If you need to trouble shoot or resend a message, you have a clean place to start.  For the outbound interfaces, I recommend that you only save the post-process transactions.  This will show you what went out the door.  If there is an error that is happening, most engine give you a testing area to see your logic applied against the message.  You can start with the inbound message and walk the transaction through the logic one step at a time. 

In regards to retention, my general rule of thumb is six months on the server.  This can vary depending on daily amount of transactions being processed on a specific server.  If you have SANS capabilities, I recommend that you keep a subsequent six months of transaction for lab transactions.  It is not uncommon for a providers office to ask for a result from a year ago because the patient is now being seen for the yearly physical.  This allows for the message to be resent without the lab having to un-finalize then re-finalize the message which is not favorable to TJC and CAP.