A prisoner is eligible for 54 day a year(85%) for good time after his first year. That is why you often see defendants sentenced to 1yeaer and 1 day.
You may also receive a reduction for providing substantial assistance to the government under Rule 35 or a 5K1.1 motion
I have add a link to my web page on products liability which will give you more information about the topic. These cases are difficult and expensive to prove and unless you have serious physical impairment and/or hundreds of thousands of dollars in medical bill, a lawyer cannot justify bring a lawsuit.
It is clear that anyone under 21 drinking at your house and later causing injury would expose you to a civil suit. Sense he is over 21, it is not as clear but would depend on the specific facts.
I would put your homeowner insurance on notice of the accident and consult an attorney before giving any statement.
We have handles case on both sides.