You would think this would be an area about which the law is clear, but apparently it is not. There is federal law which applies (part of HIPPA and Medicare participation) as well as statutes which vary by state. Ask five lawyers for an opinion and you get six answers. What I have most commonly heard is "technically 7 years is enough, but I would recommend 10".
I am obsessive about keeping old records. Not so much for legal reasons; just because I am obsessive. So when we switched to AC, I decided to scan all of them. I hired a student, basically for minimum wage, and he is now finishing the job. Roughly 16,000 charts which filled a wall of shelves, several file cabinets, and an assortment of boxes squirreled away now occupy about 20 gigs on a hard drive. Call me nuts (and my family and staff does, at least on this issue) but I like having it all. It is helpful for patient care, and frankly satisfying to recycle all that paper, save all that space, and still have all the information readily accessible.
No more "catacombs", crawling into basements or closets, sweeping away cobwebs, lugging heavy boxes around, etc. And all of those things do have some cost, which partially balances those of scanning.
