How long would it last battery on power inverter?

I have a trip coming up and the vehicle I'm using does not have a 12 volt plug and I cannot plug into the vehicles batteries (company vehicle). I have an 800 watt power inverter that I plan on using connected to at least one 12V 7AH battery. I will be powering a laptop with a power consumption from 85-100... show more I have a trip coming up and the vehicle I'm using does not have a 12 volt plug and I cannot plug into the vehicles batteries (company vehicle).

I have an 800 watt power inverter that I plan on using connected to at least one 12V 7AH battery. I will be powering a laptop with a power consumption from 85-100 watts. My question is: how long will 1 battery last on a 100 watt draw.
Update: They are valve regulated sealed lead acid batteries with a 13.3-13.5 volt output. I can have up to 6 of these batteries in parallel. I can't take a car battery or deep cycle battery with me.

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