I guess that's my cue to chime in.
Yes, unfortunately, we can legally (by the FARs) go up to 16 hours of duty in one day. And we can legally (by the FARs) fly 6 days in a row without a day off. But it is much more complex than that.
But, in most cases, the 121 carrier's crew rulesare more restrictive than the FARs, just to prevent breaking the FARs. At my company, we cannot go more than 15:30 in a day.
The rest requirements are where it gets fairly complex. They are all hinged around how many hours you fly in a constantly moving 24 hour period. As a 121 pilot, you are required to get at least 9, 10, or 11 hours of rest (rest being defined as not on duty), but that can be reduced to either 8 or 9 hours, depending on how much you fly in the rolling 24 hours. But if you do not get the required rest and are on the reduced rest, the next day you must receive a longer rest period (10, 11, or 12 hours) within 24 hours of beginning the rest period the day before, all within the 24 hour rolling day.
Yes, that last paragraph is confusing and complex. Unfortunately, flying in the 121 world, there's more to know than just how to fly and how the plane works.