I'd think like most forms of poisoning it'd take some time before death, so no it wouldn't be immediate. An immediate death would most likely be merciful, given the alternative typically is a slow suffering death.
The thing that would kill your dog (or cat) is Theobromine in the chocolate, but the amount of that in chocolate would depend on the type of chocolate. Going by wikipedia an average-sized dog would succumb to Theobromine poisoning consuming around 9x 1.55oz chocolate bars.
