What is the minimum mass required for a star to burn fuel rapidly, lasting only a few million years?

At least eight times the Sun's mass.

The lifespan of a star, particularly the duration spent in the hydrogen-fusing main sequence phase, is intrinsically tied to its initial mass. Stars that begin their lives with significantly greater mass, specifically those possessing at least eight times the mass of our Sun, have far greater gravitational pressure in their cores. This increased pressure forces nuclear reactions to proceed at a dramatically accelerated rate to generate sufficient opposing outward force. Consequently, while a Sun-like star may sustain itself for billions of years, these heavyweight stars consume their nuclear fuel supply at such an astonishing rate that their main sequence lifetime is drastically shortened, often lasting only a few million years before moving onto later, more destructive evolutionary phases.