my understanding is there is NO SOL on Student loans, you have them until you pay or die.
It varies by state.
There is no uniform, standard answer.
Some states set the begin of the SOL period as the "date of cause of action," meaning the first date that you became in violation of the terms of your account agreement, and were thus subject to civil action for default on that agreement. That is usually the same date as the DOFD.
Other states set the begin period as the date of any last payment, or a "firm offer to pay."
If in doubt after reading the legalese in your state statute, consult with an attorney in your state. They know the ropes, and can advise of the interpretation that has been established by the courts in your state.