When Very Low-Mass Stars Settle Down

The minimum mass a star can have is roughly 0.08 times the mass of the Sun. A lower-mass object would be classified as a brown dwarf. Very low-mass stars (VLMS) and brown dwarfs have very low luminosities, making these objects difficult to detect. Furthermore, it can also be difficult to distinguish whether an object is a VLMS or a brown dwarf. It can take a long time for a VLMS to settle down and enter the main sequence (i.e. a state of steady nuclear burning).

A study shows that a VLMS with 0.08 times the mass of the Sun is estimated to take ~350 million years to settle on the main-sequence where it will shine with only ~1/52,600th the Sun's luminosity. A VLMS with a slightly higher mass of 0.09 times the mass of the Sun is estimated to take ~56 million years to settle on the main-sequence where it will shine with only ~1/4,290th the Sun's luminosity. In fact, a VLMS, depending on its mass, can take as long as a billion years or more to settle on the main-sequence.

Reference:
Auddy et al. (2016), "Analytic Models of Brown Dwarfs and the Substellar Mass Limit", arXiv:1607.04338 [astro-ph.SR]