https://www.efanniemae.com/sf/guides/ssg/annltrs/pdf/2008/0816.pdf

In 2008 fannie said you may have to wait twice as long and you still might have to prove a hardship.
Before you Walk Away from your house you should compare short sale vs loan mod.

1. Will you owe for the some or all of the remaining loan balance
2. Credit effects.
3. Tax effects.  This is going to become an issue again as short sales and foreclosures may start bumping up against the expirations of the Mortgage debt forgiveness acts in CA and Feds.