Really it is impossible to characterize the battery life usage without knowing the details of the usage.
There are a myriad of factors that dictate the normal interval for rundown.
What are you running?
What is the brightness level?
Adobe Flash running?
Memory, SSD, or CPU intensive?
External drains (External Superdrive, Flash drive, other USB device)?
Keyboard Brightness setting?
The things that make your system temperature rise, are the same things that are creating a higher amperage draw on the battery. Hence, less run time on battery.
I "normally" get between 1.5 hours and 7 hours of life from a single charge. It is completely dependent on the amount of demands my workload is putting on the system.
It could save you the time and effort of a visit, if you were to give more details about your situation.
At 28 cycles you shouldn't be anywhere close to reducing battery capacity so that shouldn't be the issue.
It is possible, with heavy use, to see that sort of battery life (say running multiple browser Windows, each with flash going, downloading multiple files in the background, screen at full brightness and maxing out power demands with items on the USB ports powered by the machine).
In my normal usage my 13" Air will normally run beyond 5 hours on battery easily, but I don't do virtually any flash, don't keep my screen terribly bright and often have nothing hooked to the USB ports, using it mainly for writing and spreadsheet work.
But that's my usage and clearly battery life on all laptops will vary dramatically based on how you use the computer. If I had it ripping DVDs from an external Superdrive and then transcoding the video I'd expect to see a lot less battery life.