While I don't use SQL Backtrack for Informix, I use it for Sybase. I doubt they 
are very different. I have heard others recently make comments about the 
product, basically that it isn't very good, and that is the truth. I still 
prefer to dump my database to a filesystem.

My problems with the product include: 
it is not very robust
it is awkward to use, no GUI

Anyway, it DOES work for me. 

Expiration
*------------------
I let SQL Backtrack manage the expiration.
I have a SQLBACKTRACK management class:
adsm> q MGMTCLASS nisa_dom standard sqlbacktrack f=d

            Policy Domain Name: NISA_DOM
               Policy Set Name: STANDARD
               Mgmt Class Name: SQLBACKTRACK
          Default Mgmt Class ?: No
                   Description: SQLBACKTRACK MC
    Space Management Technique: None
       Auto-Migrate on Non-Use: 0
    Migration Requires Backup?: Yes
         Migration Destination: SPACEMGPOOL
Last Update by (administrator): PURDYM
         Last Update Date/Time: 07/28/99   10:03:52

adsm> q COPYGROUP nisa_dom standard SQLBACKTRACK f=d

            Policy Domain Name: NISA_DOM
               Policy Set Name: STANDARD
               Mgmt Class Name: SQLBACKTRACK
               Copy Group Name: STANDARD
               Copy Group Type: Backup
          Versions Data Exists: No Limit
         Versions Data Deleted: 0
         Retain Extra Versions: 0
           Retain Only Version: 0
                     Copy Mode: Modified
            Copy Serialization: Shared Dynamic
                Copy Frequency: 0
              Copy Destination: DATABASE
Last Update by (administrator): PURDYM
         Last Update Date/Time: 07/12/99   16:01:34
              Managing profile: 

The trick is setting Versions Data Deleted, Retain Extra Versions, and Retain 
Only Version to zero. Then let SQL Backtrack manage the expiration.