Fascism in Portugal lasted from 1933 to 1974.

In Spain it lasted from 1936 to 1975.

Fascism did fail (specifically, in Portugal there was a Revolution and in Spain the Fascists just gave power away whilst keeping their loot) but it took about 50 years in both cases.

Had Hitler and Mussolini not gone to war with basically most of the World, I wouldn't be at all surprised if their regimes hadn't lasted until well after both died (which was what happened in both Portugal and Spain - those regimes only fell a while after after Salazar and Franco were dead).

Meanwhile just to give you an idea of how bad things can be under Fascism without people rebelling, Portugal during the Fascist days was a dirt poor country with mainly subsistence agriculture which even received Food Aid from the rest of Europe and this without even having gone through a war. Just a reminder that during the Age of the Discoveries, both Portugal and Spain were for a while (about a century) the pinnacle powers in the World to the point that they divided it in half between themselves (which is why in South America Brasil speaks Portuguese and the rest speak Spanish and why most Portuguese "colonies" were in Africa whilst Spain was mostly present in South America), so it's not as if those weren't rich countries at some point.

So, yeah, you're right that Fascism always falls (then again, so does everything else), but it can take a while to get there and entire generations can suffer horribly before things change.