Well there is a 3 hr DOT limit for domestic flights, however there is no DOT defined time limit for international flights.

There are policies and procedures in place to deal with this kind of situation, and at certain time intervals the crew should determine if any passengers wish to deplane and not continue on the flight, and then coordinate with ground personnel to make that happen.

Course in a blizzard neat policies and procedures don't always function the way they are supposed too, but 7 hours seems extremely excessive.

Edit: Course there are always exceptions that include safety or security reasons, which includes weather.