"...analysed how soon humans might use up the solar system’s most accessible resources should space mining take off. They found that an annual growth rate of 3.5% would use up an eighth of the solar system’s realistic resources in 400 years. At that point, humanity would have only 60 years to apply the brakes and avoid exhausting the supply completely."
So then obviously we would need to find a way to expand beyond mining the asteroid belt. The article says "...humans might struggle to mine the sun, or extract useful materials from Jupiter, a gas giant with more mass than the rest of the solar system’s planets combined..." So those would be the obvious next targets, assuming we can't or don't expand beyond our solar system.