The credit applied is $5 for every month of live remaining, rounded up. So say you have 4 months and 12 days of LIVE left, they would credit you for 5 months or $25 off of the family plan (which is $99.99 for the year). Any secondary accounts you add to
the family plan get their time converted to a 2 to 1 (rounded up only if a single month is left) ratio for the whole family. Therefore if you add someone to the plan who has 5 months of live left, everyone in the family plan gets an additional 2 months. I
believe the cap is 36 months on the family plan.
Tons of information can be found here:
www.xbox.com/.../FamilyPackFAQ