It isn't clear what is going on here. My first concern is that the wall wart is not capable of charging the battery very fast so I'm wondering if you are simply not starting with a fully charged battery. I'd check the voltage of the battery when the wall wart is charging it to see if the voltage from the wall wart is high enough too.
What is the current or power draw of the load you are putting on this thing? Is the inverter a store-bought one or something you built? I just don't understand your claim that 55 minutes is far below the AH rating of the battery.
I think that you can expect the amount of charge you get out of the battery to approach its rating, even at relatively high draw, and anything less than 70% of rating would surprise me. If you want to learn more, why not have a look at an SLA battery manufacturers website for data on the effect of discharge rate vs battery capacity?
So, assuming you are getting, say, 80% of rated capacity at a discharge rate of C, then you should get about 69 watt-hours, meaning that your load is pulling about 531 mA at 120 V (nominally about 64 watts). This assumes an inverter efficiency of 85%.