Short answer, not defined yet.

> Hellboy ages very differently from humans. In the story *Pancakes* he is
> two years old but appears to be somewhere between 6 and 10 human years
> old. In *Nature of the Beast*, set in 1954, the ten-year-old Hellboy
> appears fully grown. His rapid physical maturation is in contrast to
> his actual rate of aging however, which seems to be much slower than
> humans. Throughout the sixty-year span of the comics he does not age
> beyond the plateau of physical maturity. This mystical aging process
> is similar to the other demons and supernatural beings that populate
> Hellboy's world. The lifespan of a Demon or Half-demon (Hellboy's
> mother was supposedly a human witch) is left undefined within the
> comics, and seems to range from decades to many thousands of years.[(source)][1]


  [1]: http://en.wikipedia.org/wiki/Hellboy#Powers_and_abilities