From personal experience, it’s Awesome when you have a client in their 50’s that you started working with in their 30’s, and you can show them how their 50 year old self could run circles around and kick their 30 year old self’s….
So forever, of course! Because maybe the client I’m working with today will look me up 10 or 20 years from now and I’ll want that old information to refresh my memory and discuss with the client – whether they have worked with your periodically for years or come back to you after years of hiatus.
This doesn’t mean you have to keep a record of everything (billing info, etc) or paper record of anything. I scan only pertinent old client paperwork into my computer for storage for safe keeping… anything pertaining to “liability” documents or training assessments and sessions; the rest I discard as appropriate.