Perceived volume and power are not linear. Volume plays a huge part of the equation...if you listen at 50% volume, you're using ~20x less battery. If you crank it, you'll get a couple hours, if you drop it to half you'll get two days. Drop it to 1/4 or less and you'll get a week...give or take.
The point is your 1000w amp is using very little power if you're not cranking it, and everyone's perspective on volume is different. My wife hates shouting over the stereo, so when it's chat time the volume is around 5-10% and I can run two weeks at that volume. But when we're partying, it's around 70% or more, and we're only good for 3-4 hours. At full volume we're good for about 30 mins.
If you crank it up, your 1000w amp on a group 27 100ah battery is good for about 10 mins or so. Obviously you're not cranking it all the time.