Ummm Cubans are more Spanish than native. Most of the indigenous Cubans died and Spanish colonists went there, this is diffrent from Mexico and the rest of South America the islands in the Caribbean fucked them over. So it doesen't really count as colonialism and cultural assimilation any more than what the British did in North America the people did not assimiliate they were already of that culture.
And both are commonly recognized cases of colonialism.
The time limit is a generation also Kalingrad/Koiningsburg is owned for like 80 years The got it in 45. From 1940 to 2020 is 80 years.
40 years, 80 years, whatever. The question is - are we talking law, pragmatism, justice? In either case, why would "legitimate ownership of the land" be a question of essentially a national game of King of the Hill, where a country can just invade with no good justification at all, but if it manages to take the place, settle it, and hold it against potential counterattacks by the other side or its allies for X decades, whether X is 4, 6, 8 or 10, suddenly everyone is supposed to recognize them as the legitimate owner and recognize their ownership of the place and in fact be outraged at the previous owner for continuing to want it back?
I mean Kalingrad should be demiltarized, there isn't a legitimate reason to have armies there as the area is not defensible with conventional means. But it could be used as a jump off point to invade. But beyond that America's actions in the Cuban missle crisis kinda disproves your point. Unless you are saying that the U.S. should have been ok with missles in Cuba?
No, it wasn't "missiles", missiles aren't made equal, it was Cuban nuclear ballistic missile crisis. Those missiles are the ones that got all the press, but not many people know that Soviets have kept a whole bunch of troops and even some combat aircraft for decades after the crisis.
(Archived document, may contain errors)
www.heritage.org
So if anything, if you want to use Cuban Missile Crisis as the yardstick, it has established nuclear ballistic missiles as the red line, not general military presence.
Specifically in threateningly placed third party countries, because if we are talking technically own island/exclave, then Russia is already crossing it, keeping such missiles in Kaliningrad, and possibly also Crimea.
And France, Germany, and Russia will build a land bridge to the UK to remove the power of the Royal Navy.
Calling it irrelevant is just dismissing it with no reason, international law is more of a wink and a nudge and we can speculate based on how things are now. After all why should man made islands be treated diffrent from ones made by God? So China creates an islands in the pacific and Atlantic right next to the US changing geography to remove the greatest boon America benefits from two moats seperating it from the other great powers in europe and asia.
You're the one who started with China building UK sized islands in the middle of deep Pacific. This is science fiction. Of course international law and custom are not designed to handle situations that require 40k level industrial technology to make happen.
Just like existing laws don't handle interstellar warships, unaging supersoldiers and psionic power users.
Meanwhile, natural islands, well, they were there, they were up for grabs by whoever shown up, if you really cared about it, should have claimed and settled it first.
If we go back to reality, building islands is something that can be used for little more than legal trolling and small to medium military bases, on reefs that already have minimal depth.
And yes, there are some international laws related to it.
Within the short span of a year, China’s rapid construction of artificial islands in the disputed Spratlys has radically changed the geographical and security landscapes in the South China Sea. This island construction has so far created over eight million square metres of real estate in the...
amti.csis.org
For one if you do it within the most extended EEZ, the country in question may send their navy to ask wtf are you doing there.
But besides that, environmental law and so on, it's down to economic zone and territorial water disputes, UN, courts, and navy shenanigans too.