Here is a worthwhile article, despite some misconseptions about legal malpractice.  The article states:

"As for risk management, the statute of limitations on legal malpractice claims generally begins once a client discovers possible negligence and runs for years thereafter, depending on your state, which could expose a lawyer and its firm to claims well beyond a file’s normal retention period. Storing client files electronically makes longer retention periods more cost-effective and long-term risk management more feasible.

This is wrong in NY, as there really is no discovery statute of limitations. While there are exceptions, one should not depend on any date later than the last day the attorney represented the plaintiff. 

In any event, the rest of the article on paper retention and electronic files is worthwhile.