Spec'ing an always-on alarm/tracker for a car battery
Battery capacities are usually provided in amp-hours or milliamp-hours. A quick search shows that typical car batteries have a capacity of around 45 amp-hours. That means with a one ampere load, it has enough energy to run for 45 hours. This isn't the most accurate way to think of capacity, since the total energy available depends on the manner in which it's used (quickly? slowly?) but it should suffice for some back-of-envelope calculations.

So to estimate the runtime, divide the capacity in amp-hours by the average current requirements of your device. If your device requires 50 mA on average, then:

\$ 45Ah / 50mA = 900 h \approx \text{37 days} \$

Most car batteries are not designed to be deeply discharged as part of normal operation, so avoid that. It's hard to say if your car has any sort of protection against running the battery down enough that it won't have enough to start. It would depend not only on the car, but also on which circuit the device is connected; some may have protection while others may not. I know my car does not offer any protection (even the dome light will run the battery down), but as the car in question gets newer and more expensive it becomes more likely there will be some circuitry that will do something intelligent.

If your device has batteries of its own that will power it should the main power be lost, then you can simply add that runtime to the estimate runtime of the main battery.

It's hard to say how long it will take to recharge the battery. It will depend on the charging method (fast charger? slow charger? the car's alternator?) as well as the type of battery, but something an hour and 2 days is likely. See this page from Interstate Batteries for more information on that.