Customer interactions are usually considered Records that will need to be retained at least 5-6 years. In general, on-line text chats that are considered Records are asked to be recorded in either an application or copy into an office document and stored in a Record Management system.
Additionally, regardless of whether it is defined as a Record or not, these interactions are tagged with key words and categorized in our knowledgebase, which is a resource open to everyone in our organization for their own research and inquiry.
Learnings for us include .... take sometime upfront to set rules on what satisfies as a online text chat Record, have someway to spot check or QA the country metadata so appropriate regulations / retention can be applied correctly, and widely publicize your knowledge base, not only during the initial kickoff and implementation but also throught out the sustaining organization to enhance usage rate.

I suggest 3yrs unless you identify a regulation on your business that is longer. Keeps system cost down and you should be able to identify trends within weeks/months. The rolled up trend data could be kept longer but globally, retaining customer PII information is a hot button. The newer ECM systems are worth a look now as they are becoming pretty good at reading/understanding concepts and can learn to quickly identify issues/trends. They may be able to show trends within days if enough calls come in.