How long would the charge from a 12v car battery last using a 750w device using an inverter?

No inverter is perfect, so there are some losses. Usually the larger the unit, the higher the losses but they are still fairly efficient. So the amount of power consumed depends on the device consuming the power NOT the inverter. For the life of a charge, I have car batteries from 35 amp hour ( a small hatchback) to 120 amp hour ( a large 6 cylinder car.) So for convenience take a 60 amp hour rating. The energy in the battery is 12 * 60 = 720 Watt hours 750 W * x hrs = 720 And you can see that x is a little under 1 hour. However for SOME items the 750W is a maximum not an average so you may find that it will last longer than this figure.

Hello Antony G, yes definitely as you use higher wattage then the battery would get drained quickly. The size of the battery is termed in Ah ie ampere hour. The car battery would be of 40 Ah or 60 Ah. As one uses 750 W inverter it would be advisable to use a battery with capacity 130 Ah.