String, piece of, how long? I've had to do that more than once and can tell you that an old points-ignition system that draws 4A will exhaust a battery in 6~10 hours, depending on condition. Electronic ignition isn't too different, really, in many cars might be more or less power-hungry, depending on design.
Diesels traditionally used a mechanical stop lever on the pump (operated by a cable), so once they were running, they'd run until the tank ran dry. Diesel cars of up to ~12 years ago had a simple electrical stop solenoid, which would draw a couple of amps, so on a par with the old points ignition, if that's all that's running. Modern diesels will draw a lot more, with their extra pumps and kit and caboodle, and I'd not be surprised if the current draw is well above 10A. Of course, some modern cars have electric power steering, so all bets are off, even if it only works on the non-ahead position, it's still a substantial drain and the difference it makes to the journey length might be enough to leave the car stranded. You don't really have a choice over the use of the engine cooling fan either, and there are a lot of circuits drawing current on a modern car, compared to the bare necessities that you could get away with.