US rules vary depending on the type of operation (121 Domestic, Flag or Supplemental, 135 Scheduled or Un-scheduled), number of pilots and number of flight crewmembers. A two pilot domestic crew is limited to 8 hours flight time in 24 with 9 hours rest (or it could be 8 hours or 10 hours depending on how long the next rest period is scheduled), 30 hours in 7 days (with one 24 rest period during those 7 days), 100 hours in a calendar month and 1000 in a calendar year. A supplemental crew with two pilots and a flight engineer flying outside the 48 contiguous states is limited to 12 hours in 24, a 24 hour rest period every 7 days, 120 flight hours in 30 days, 300 in 90 days and 1000 in 12 months.
There are no simple answers.