I am getting an inflatable boat and I am going to put a 12 volt trolling motor on it, so I don't have to row, row, row my boat ;D.  My question is how long will the charge last on a 12 volt battery?  Do I need to get two for a day of fishing or will one last all day?  I know it has a lot to do with how much I use it during the day, but all I am asking is in general should it last for 4-6 hours on the water?

Basically this depends on two choices that you will make. The trolling motor, how many pounds of thrust = how many amps it will draw, and the size of the battery = how many amp hours it will deliver. Also the trim of the boat ( fore  and aft balance) is important. Don't have all the weight in the stern. You can use heavy jumper cables to keep from having the battery almost in your lap because of the short cables that are attached to the trolling motor.

I would think you could get at least 4-6 hrs. if your boat is about the size of a 12-14' john boat.  And not planning on running white water.  

Honestly, I can see me never using more than the 1st or 2nd speed on the motor. I basically want to use it to from drifting, I fish on small county lakes, 20-30 acres at most. It is a 30 lb thrust Minn Kota.  

u should get a deep cycle battery. I have a normal truck battery, and i'm always charging it

The deep cycle battery is built heavier than the trucks/ auto starting battery.  The starting battery is not designed to endure the full discharge/charge like a deep cycle is suposed too.  The life of the starting battery will most likely be much shorter, along with the charge rate.

A deep cycle battery is designed for trolling motor use. That is a relatively small amount of current ( 30-40 amps) for a relatively long period of time ,hours not minutes. An automotive or marine starting battery is designed for a a much larger amount of current to be used over a period of several seconds,just long enough to start an engine.  The deep cycle battery is the ONLY WAY to go for a trolling motor. You should have no trouble at all fishinf five or six hours on the small lakres you are considering.

A deep cycle battery is designed for trolling motor use. That is a relatively small amount of current ( 30-40 amps) for a relatively long period of time ,hours not minutes. An automotive or marine starting battery is designed for a a much larger amount of current to be used over a period of several seconds,just long enough to start an engine.  The deep cycle battery is the ONLY WAY to go for a trolling motor. You should have no trouble at all fishin  five or six hours on the small lakes you are considering.