A cama is a hybrid between a male dromedary camel and a female llama. The first cama was born on January 14, 1998, yet on the Wikipedia page it is said that a cama's life span is 30–40 years. How was that number determined? Is it simply the average between a camel's and a llama's life spans?
Since no source is given for the 30 - 40 years estimate in Wikipedia, we can't find out how the authors of the Wiki page reached that estimate, but 'someone made an educated guess' seems likely. There are a few reasonable ways that one might educatedly guess the longevity of the cama, but there are good reasons to treat those educated guesses with caution.
First, as you suggested, you could just estimate the cama's longevity from the life expectancy of camels and llamas. This is likely to fall in the right ball-park, but should be interpreted with caution: ligers (lion - tiger hybrids) are reputed to have high rates of premature death, and the same may well apply to camas.
Second, you could plug the animal's body measurements, metabolic-rate measurements, or similar measurements into a model which relates species' attributes to a measure of their longevity. Many different models of lifespan have been built (e.g. here), and if your goal is to get a rough estimate of how long a member of a particular species will live, those models are not a bad way of making a first guess. The 'this is a weird hybrid and might have issues with premature death' caveat still applies, though.