The "clock starts running" from when you defaulted on your mortgage (failed to make payment) or, as you put, became delinquent. That (failure to pay) is the breach giving rise to the cause of action (grounds to sue). Your state, unfortunately, has perhaps the longes statute of limitations, or time period during which you can be sued, for situations like this: 15 years (it's the written contract statutte of limitations). So you could be sued for up to 15 years from when you first failed to pay.