Black holes can exist with any amount of mass, as far as we know. You can calculate the size of the event horizon for any given quantity of matter. IIRC, even the Planck Temperature is the limit to our current understanding that it is because a photon emitted by an object that hot would be energetic enough to form a little tiny black hole.

This graph shows a few things about black holes in a way that I love.

1. Black holes do have a lifetime and are not eternal. I think somebody already mentioned Hawking radiation.

2. Extremely small black holes actually have a pretty short lifespan. The math says they glow brightly and basically explode as they get close to fully evaporating.

3. Black holes with any appreciable mass last a REALLY fucking long time. A black hole with only the mass of the Red Sea or Greenland would last many orders of magnitude longer than the longest-lived red dwarfs, which themselves go for trillions of years. An online calculator tells me that the Schwarzschild radius for 10^20 kg (roughly greenland on the graph) is 1.4852e-7 m, so sub-micron in size. And though it will not last forever, it will still be virtually unchanged in trillions of years after every current star burns out.

I also hit up Wolfram Alpha's black hole lifetime calculator. For 10^20 kg it gives me 2.667 x 10^37 years. For one in the range of metric tons or less, you are looking at nanoseconds or faster. For a black hole to last longer than a second, you need to be up around a quarter of a million kilograms.

So when I said they are supposed to basically explode as they get tiny and evaporate? The most powerful hydrogen bombs convert a couple kg of matter to energy at most. A 250,000 kg black hole converting entirely to radiation in just a second must be some crazy shit.