OK, one last time - How much mass before star-fusion happens

I keep reading conflicting reports when the topic is brown dwarfs and
gas giant planets.

Have astronomers finally settled on how massive an object must be
(assuming it is mostly H and He) before the star "ignites". And how
long will it "burn". Being just on the verge of stardom, I'm thinking
that they wouldn't last long, yet I know that the more massive a star
is the faster it "burns out".

Enlighten me someone.

Thanks.