The short answer is that no-one really knows. If an extremely well-preserved skeleton of a triceratops is ever found then palaeontologists may be able to come up with a more accurate number by analysing the growth pattern of the bones, although this will tell us far more about how long they took to reach maturity rather than exactly how long the lived for.
Until that happens we are left with only speculation as to Triceratops' lifespan.
Some have tried to use the growth patterns of modern reptiles as a guide. Generally the larger the reptile the longer the life span, so this would imply a VERY long lifespan for the 6-12 ton triceratops (possibly 200+ years). However this reasoning may be unsound as the metabolism of a 6-12 ton creature is possibly very different to that of a 1 ton crocodile.
Estimates for larger dinosaurs such as the brontosaurus put their life expectancy at around 100 years. A triceratops is only 1/3 the size of a brontosaurus, which could mean its life expectancy was less.
Also to get a meaningful estimate we would need to know about the lifestyle and habitat of the triceratops. Even if it is determined that they took (as a wild guess) 15 years to reach maturity, possibly most of the adults were subsequently killed and eaten by Tyrannosaurus Rex. (It is definitely known that the Tyrannosaurus Rex preyed upon Triceratops.)
So the basic answer is that no-one knows. But it can be fun to try and guess.