The amount of fluctuation you are seeing is fairly high. Let me explain how the time estimate works and we can go from there. Your Mac knows "about" how long the battery will last. It calculates this based on how hard the machine is working. For example, if it's just sitting there idle, you may see a six hour estimate. If you start running something that uses lots of processing power, it may instantly drop down to three hours. If you use the car analogy... if you're just coasting along at a steady speed, you may get 20 miles to a gallon... however, if you suddenly find yourself climbing a steep hill and you need to really press down on the gas to climb it, your car is going to use a lot more gas... so it will run out faster.
What the time remaining shows is an estimate of how long the battery will last if you leave the system just as it is. As soon as you start running programs and using more power, it adjusts the remaining time accordingly. For example, if you're just reading a plain text web page, you may see 7 hours remaining. Your computer says "Hey, this is easy, I can do this for 7 hours". However, if you decide to start playing videos or games, your computer will say... "Hey, I'm having to work a lot harder now... I this pace, I'm only going to last three hours".
So, depending on what you are doing at any given time, that number can fluctuate greatly. A better way to monitor your battery is to monitor the percentage (just click the battery icon and select "percentage"). This will show you the "actual" percentage remaining. It won't fluctuate like the time remaining estimate.... it will just go down faster under a heavy load and it should never go up (until you start charging it).
The fact that you're getting about 7.5 hours would seem to indicate your battery is working fine.
