First, I would advise that you change your service agreement to provide that you WILL destroy records in your possession after X years, and if the client/ex-client wants anything, you will provide it to them prior to X years (for a fee).
Second, for your own protection, I would recommend keeping everything forever. One never knows when a client or a participant or someone is going to file suit, and put you in the hot seat for something. Relying on a "statute of limitations" defense is a last resort. Relying on proof you did nothing wrong is much better.
Finally, ERISA requires the PLAN to maintain records for so long as necessary to calculate the benefits due (regardless of how long that is). Fiduciaries should keep everything forever - but that doesn't help you.. Who owns the records? Is it "plan records" or your records or what? You need to answer that before you can make a decision about what and when to purge.
After all of that, the best guess is that the stature of limitations runs out six years AFTER the termination of the relationship. I'd put a buffer in that of a few more years, at least (one never really knows when the relations completely ceases to exist - e.g. providing data to the new provider to do the next 5500 may be after the "normal" end of the relationship - but may still be considered part of the relationship)).