how much power

How many licks does it take to get to the center of a tootsie pop?  I'd say both questions will have similar answers.

BTW, when the vehicle is running the amplifiers are supplied power from the alternator, not the battery (assuming the electrical system isn't overtaxed).  For the most part the purpose of a battery is to provide power when the vehicle is off, and to provide power to start the vehicle. 

ok sorry guys let me rephrase...

So, i an installing a system into my wakeboarding boat. My total system will include a 4 channel 60rms @4ohm and a 4 channel 75rms amp and a mono 300rms @4ohm... and a deck but i dont think that is a very big draw. I am running a 4aug cable to a dist. box (both for ground and +) and using 8aug cable to each amp. I understand the alt keeps the battery topped off while the engine is running but my consern was for when the engine is off. I honestly dont know what brand battery i have. I hope that helps.

I understand you are asking about your boat and I will answer that question in a second, but I do want to make one point clear.  You said:

No, the alt in a car of truck supplies ALL the electrical loads of the car, the engine, and any additional things you've added like sound systems, AND keeps the battery charged.  The battery in a running vehicle will not be used unless the alt cannot keep up with the load, and then it indicates you need a larger alternator.  I just don't want anyone reading this thread to get a wrong impression.

As to your battery load question, you need to know the ampere-hour rating of the battery (NOT the crank amp rating) and the total ampere demands of your system.  For example, if the battery is rated at 40 Ah and your total system load is 40 amps, the battery should sustain this load for one hour.  Assuming it is brand new and fully charged.