Battery & Converter for AC amp

[battery777]

Bottom line up front... How do I use a battery to power an AC bass amp?
I was hoping someone has tried hooking a normal DC to AC converter to a battery (I'd use a deep cycle, like a boat battery). My converter is 300W continuous/600W peak, and my amp says 180W, so I'm pretty sure the power is ok, but I'm concerned about the Hz. Of course AC is normally 60HZ, but when you convert DC to AC, I know it's not "true AC". What I don't know is "Is this bad"?
The other question is how long can I expect a 12V battery to last? It will be used in the Pit for the marching band at games, so not constantly playing, but probably at pretty high volume.

Thanks to all for your input!