There are many different ways that it works. For a standard part 121 airline (Delta, NWA etc). The legal limits are as follows:
100 hours a calender month, 1000 hours a calender year.
They also need I believe one out of every 7 days off and are limited to 30 or 32 hours in any 7 day period (depending on domestic or international). Keep in mind there are a number of different time requirements for various types of part 121 operations. Most airilines have union agreements giving them better work rules than the FAA mandated minimum.
Generally they will be working rotations... 3 on 4 off and so on. They are also taking reserve times (on call) depending on seniority.
I fly for a part 121 airline, but we have an exemption to the normal part 121 duty requirements (apparently flying in Alaska is harder, so we must be better and thus need less rest) and even though we are a scheduled part 121 airline we follow part 135 unscheduled work rules.
Our rules are as follows:
500 hours a calender quarter,
800 hours in two consecutive quarters,
1400 hours a year.
We also need 12 days off a quarter. This means we can work over 5 months without a day off and still be legal. Fortunately that doesn't happen (but could). I recently worked 3 weeks without a single day off. We have had guys with in excess of 160 hours in a month recently. The highest I've heard of is just around 200 (it's tough to get above 200). The FAA rules are what we go by. In both cases pilots are allowed up to a 14 hour duty day, and 10 flight hours per day as a crew. A string of 14 hour duty days, or worse yet 10 or 12 hour duty days with minimal rest (changes your sleep pattern each day and may not allow for a full 8 hours of sleep) will get you worn out.
I would like to point out that you will be hard pressed to find many "Commuter airlines" these days operating under part 135. There are fewer than 10 in the country, and most of those are in Alaska. ASA, Skywest, Comair, and all the other operators flying turboprops and regional jets are part 121 carriers and are governed by the same part 121 work rules. The main difference being their union agreements tend to not have as good of work rules in them as the major carriers do.
Great Lakes, which flies Beech 1900s does have their own part 121 exemption, it gives them 1200 hours a year as opposed to the standard 1000.
Different companies will have different amount of days you work. I usually work week on, week off. So I'm working half of the days in a given month. But I have had stretches of much more than that.
At a major airline it might be 3 on 4 off or so on. Keep in mind that the "on" days with most carriers are 24 hours a day of being "at work." You aren't really home all that much more or less than a 9-5 job, it's just that your time home happens in blocks.