No, it's not that efficient, but it's convenient - especially if the appicance requires a regulated external power source to charge internal batteries. If the appliance comes with a 12V lighter-socket adapter, then by all means use that instead, but it might be cheaper to use the invertor than buy lighter-adapters for every bit of kit you want to power.
They aren't that innefficient either, and the one I have claims to switch off if the voltage drops to below 11V, but I've never been able to run anything that long to find out.
For battery calculations, you need to know the rated AmpHour capacity. But note that they are usually stated at a particular amp rating. Eg. a typical car battery might be 40 Amp Hours at 10 Amps. That means it will last for 4 hours (down to some set voltage, maybe 10V, I don't know, would have to check spec.) at a current drain of 10 amps, but it wouldn't last one hour at 40 amps (usually a lot less!)
So drawing 5 amps, you might expect it to last 8 hours. Most car batteris will have their rating printed on the labels.
Gordon