If you're talking about texts, I make a regular habit of deleting them from any individual once it reaches a certain threshold # (usually after a 100 texts). So I rarely have any texts that are older than a week or 2 at most.
The situation is different for e-mails (which aren't necessarily linked to my phone) -- I have some e-mails that date back to 2010, and I've saved some e-mails in archives that date back to 2009.