What is common number of flight hours a year for an airline pilot?
Flight hours are limited to 30 hours in 7 days, 100 hours in a month and 1000 in a year.  Scheduling rules also say I need at least 1 day off a week, so 6 day trips are the max for me (121 US domestic scheduled).  

Let's look at a typical trip, which at my company tended to be a 4 day trip.  You want good productive trips, but you can't always get them.  On a 4 day trip let's say I have 6 hours of flight scheduled each day, or 24 hours scheduled that week.  I am getting paid the better of 24 hours or the actual flight time.  Now let's look at my duty day.  For a 6 hour day with 4 legs it was not unheard of to have a 12 hour duty day.  So now for a 4 day trip, I'm "at work" 48 hours and being paid for half of that.  Now consider that I'm not going home every night and my trip starts at 6 am on Friday and ends at midnight Monday night -- that totals to 90 hours away from home.

How does this compare to a 40 hour work week in an office?  In that case you are on duty 40 hours, getting paid 40 hours (or salary) and if you have a 1 hour commute one way, you are spending 50 hours away from home.

Back to the flight schedule.  A given month will have trips less productive than 24 hours and although you can probably build a 96 hour month if you tried, the average line of flying is typically going to be 75 hours of flying.  You can ballpark that will equate to 150 duty hours and 300 hours away from home for the month.  

Extrapolate that to a year, and you have 900 flight hours, 1800 duty hours and 3600 hours away from home.  I would say this represents the average US domestic 121 pilot.  Some do more, some do less, but this is pretty typical.

What do we do with all of that duty time that isn't flying?  Let's see:


Eating
Updating charts
Checking the weather
Checking the paperwork
Coordinating with gate agents, ramp agents, fuel agents to make sure we leave on time
Programming the FMS and getting our IFR clearance
Preflighting the airplane
Postflighting the airplane
Doing the weight and balance
Sleeping


Note, that all of these activities require we are at the airport, in the secure area and we are not being paid.  The pay clock starts (roughly) when the airplane is buttoned up and the parking brake is released and ends when the parking brake is set and main cabin door opened.