I'm guessing they live slightly longer than humans, given that Dukat is old enough to have had an affair with Kira's mother and still looks good in DS9. However, is it ever specifically mentioned what the average cardassian lifespan is? I'm looking for any answer, as long as it's from a published Star Trek work.
There are plenty of contextual clues regarding Cardassian longevity (the fact that Tekeny Ghemor was present at the beginning of the Bajoran Occupancy, for example) but nothing definitive in either the shows or the canon-ish novels.
I think the closest you'll get is from the Expanded Universe "Cardassian Sourcebook" which states that
"Cardassians of all races have an average lifespan of 98 years for men and 96 years for women. The highest recorded age attained by a Cardassian is 144 years".
Although these RPG sourcebooks are considered non-canon, a number of facts and figures from them (notably regarding uniforms and ranks) have been used in both the TV series and many of the other authorised fictions, including the Terok Nor trilogy
If we take Dukat as an example, then it is likely the average Cardassian lifespan is generally longer than that of current Humans but of similar length to 24th century Humans - at around 120/140.
He was in charge of the Cardassian Occupation for 23 years according to Memory-Beta, was born in 2308 according to the same source and he was still able to reliably interact and fight with Ben Sisko (who was a 42/3 yr old man) in both 2374 and 2375 where the average human lifespan was 120, so likely the Cardassian lifespan was similar to that of Humans, if not a litle bit longer.