Now, it's been said that red dwarfs are completely convective, therefore they can convert 100% of their hydrogen supply into helium. However, is this really practically true? One would think that at some point in its lifespan that there simply won't be enough hydrogen left to sustain a practical fusion reaction, because there's more helium around than hydrogen, which would make it harder and harder for hydrogen to find other hydrogen to fuse with. Just as a guess, let's say that's about at the level where helium makes up 90% of the mass or perhaps volume of the star? Is there any theories that discuss this issue?