Depends on the State.  

I keep seven years (from date of filing, including extensions).  YMMV

I also keep 7 years.  I don't recall the reason why I picked 7, but maybe it was just because 7 is a lucky number.

As opposed to 'one is the loneliest number' 😉

I do seven because of CA requirements (of course, CA is a P.I.T.A.)

Thanks for all your help,

Christopher