Life span directly relates to body mass and metabolism. Each creature (I'll attach conditions in a moment) gets about a billion heart beats in its life. For smaller creatures with high metabolism, like sparrows and rabbits, this means a very short lifespan, sometimes only a few years. For larger animals with slower metabolisms, like tortoises and elephants, this means a longer lifespan, sometimes over 100 years. Interestingly, chickens and humans are big exceptions, each of them having double the average number of heartbeats per lifetime.
So the question is: How big and how fast? If your species is small and moves rapidly, then their lifespans will probably be in the two-to-five year range. If your species is big and slow, look in the 70-to-120 year range.
With that in mind, remember that age does not the species make. With a shorter life, the individual has a higher rate of living, by which I mean that everything happens faster. A rabbit matures much more quickly than does an elephant, but both have comparable percentages of infancy- the percent of the individual's life that it spends in infancy is comparable betwixt the two. You might have a species that lives only a year, but you can still write a coming-of-age story. Likewise, you might have a species that lives for a century and a half, but you can still cover all its life in one book. The trouble arises when you try to apply human time scales to non-human creatures. Take an extreme example, a species with a lifespan of ten minutes. Within ten minutes, an individual is born, lives, breeds, and dies. You could write a novel about the individual, just as you might about a human. There could be legends of a time, many many generations ago, so long ago that it is almost forgotten, when the world was dark, and there was no light anywhere (night-time) or when all the world was blazing with light (day-time). It all depends on the time scale, which I assume will be on the scale of the species whose lifespan you are trying to calculate. As long as your time scale matches your species lifespan, it really doesn't matter, other than stuff like seasons passing or the landscape changing. It's all about percentages and perceptions.