When someone moves, how long does PO automatically forward the first class mail if someone completed the change of address form?

i just had a card I sent to a PBS member returned to sender  'unable to forward'.  I last mailed something to her at that address in DEC so if the rule is still 6  months, even if she moved, it should have been forwarded. 

First class mail & most other mail should be forwarded for 12 months.  Magazines are only for 60 days. There is also an option to pay to have mail forwarded for longer.

Here are the PO FAQs 

Also, sometimes a piece of mail will be returned to sender in error. It's possible the person has not moved at all, and the postal employee has made a mistake.

Forwarding is always very hit and miss regarding how well it actually works.