Most starter batteries are rated in CCA: cold cranking amps. The equivalent amp-hour rating is typically around 8% of the CCA. So if you have a 600 CCA starter battery, its AH rating is somewhere around 50. That means it can provide 50 amps for one hour, or one amp for 50 hours, or any combination in between. Your laptop charger probably draws somewhere around 50 watts at 12 volts. That means it draws about 4 amps. So charging it from the car battery with the engine switched off means it will charge for around 12 hours (50 divided by 4) before it depletes the car's battery. Reduce that number by 15% or so to allow for the inefficiencies of inverting from 12V to 110V then back to 12V again (or 9 or whatever the laptop charger uses). Ten hours of charging, tops. Since most laptops charge in way less time than that, you should be OK. But if you leave it plugged in for more than about four hours, you can expect starting the car - particularly if in cold weather - to be somewhat less than enthusiastic.

Starter batteries are not designed for a steady current draw over a long period of time. They are designed for a very heavy load for a very short time. So significantly depleting one multiple times could result in shorter life. If I needed to charge my laptop from the car's battery on a continuing basis, I'd be inclined to buy a car battery that is a hybrid starter/deep cycle one. Any good battery store will have two or three of those, but expect to pay $$.