The short answer is around 30 minutes a day, usually split into two walks. The useful answer is more complicated, because a four-pound dog with a delicate windpipe and a tendency towards kneecap problems has real limits that a Labrador doesn't.
This guide covers how much is right at each age, why over-exercising a puppy causes lasting damage, indoor options for bad weather, and the temperature limits that genuinely matter for a dog this small.