Unless Cardassians age more slowly than Humans, as some Trek lit writers have assumed...Dukat said that the Cardassian Union had existed for 500 years, not that the military had been in control that long. He was probably just referring to the Cardassian people as a whole.
The way Madred and Picard speak, the way the current Military government came to power about 40 years ago (I'm guessing Madred was about 50 years old, the same age as David Warner at the time).
Actually, 40 years seems too recent, since that's how long the Occupation of Bajor is supposed to have lasted. Maybe it's something between 50 and 70 years (or even 100, depending on how long we assume the lifespan of a Cardassian to be).
It's funny that so many people will name a former prominent member of the Cardassian version of Stasi/KGB (or if you're one of those fans enamoured of the Nazi Germany comparisons, Gestapo), former torturer, assassin and who knows what else, not to mention a pathological liar, as a good guy...Cardassians are diverse, like most other species. But in TNG/DS9 they were portrayed as being cruel and cold, which by and large they were. Cardassian society was based on near totalitarianism. There were persons such as Garak (who in my mind was not evil, but certainly could do bad things if he wished to or served his purpose), Tekeny Ghemor or Marritza, to balance Gul Dar'heel or Dukat, in fairness.because he's so cool, has great lines, is relatively reasonable and loves Cardassia... oh, and because he's on our heroes' side... (which he may not have been if he hadn't ended up in exile for whatever reason...)
You know, I've always loved Garak, but I find myself liking him a little less every time someone calls him a good guy.
"Honor" is just a catchphrase that the Klingons use to justify whatever they're doing, even if it's most dishonorable, just like Humans will speak of tolerance and acceptance of other cultures even while they're being at their most prejudiced and arrogant. See "Sins of the Father" and "The House of Quark" for some examples of Klingon "honor" being nothing but a cover for political expediency or greed.The fact that we see persons such as Duras and his sisters and son, and Gowron (at least in latter DS9), to balance off honourable Klingons like Worf, Kurn, Martok and Kor doesn't mean that Klingons in general aren't a culture that believes in honour.
It depends how one defines good and bad. Didn't Sisko trick Romulans into war? Didn't Worf, for whatever reason, effectively abandon Alexander after the Enterprise-D was destroyed? Are these good actions?

As for Klingons, we saw they were nuanced, and not all gave a hoot about honour. At least it made them complex, and interestingly portrayed.