In the film Hellboy, when speaking about Hellboy's appearance, the aide agent Meyer replaces quips "Think reverse dog years" and mentions that even though Hellboy was born 60 years ago, he was physically in still his 20's. According to the comics, what is Hellboy's (potential) natural lifespan?
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)
It's not stated in the comics, but we can presume that he doesn't get older after getting mature (at least not like humans do) thus there is no natural lifespan for him. It seems that he inherited immortality after his father. Besides in Darkness Calls he is said to be deathless and in two situations he is technically killed (by Hekate and Urgo-Hem respectively), but he gets better ;) He is a being of magic and natural laws doesn't seem to limit him as much as they limit humans.