Probably doesn't have to take place any number of years depending on who you are going to play as in KotOR III, if it is a new character than probably they only need to make it 2 years at most not even that. However, if they make it like Final Fantasy VIII where you sort of had two main characters....and those two characters where Revan and the Exile then perhaps it could be directly after KotOR II,
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note: I know that may not seem like a spoiler but if I didn't blacken the text then people who dont want the game spoiled could possibly infer something about the plot from the paragraph preceding it.