It's more of a percentage game than an actual to soon or to late.
The example you named, (Game of Thrones spoilers)
Daenerys' descent in to madness had a good start, just a lousy ending. (She had dictator traits from as soon as season one due to her birth right claims and marital claims she was rather fond of.)
During her journey you see her slowly become more violent and less sympathetic to her enemies. It gradually built it up during two seasons, so slow you don't see it while watching but if you go for a second watch after seeing the result you see the signs. (Like crucifying the slavers). Everybody was okay with what she did at first because she was doing it to bad people, but burning a woman alive in season 1 already showed she wasn't as innocent as she looked.
The problem is not that she went crazy, and it was properly foreshadowed, but there was a lack of triggers. In the past her madness came out of vengeance not out of malice. If, for instance, she would have gone crazy after losing Missandei or one of her children it would have worked, but those events did nothing to her state of mind. Then after she won she went crazy.
Coming back to the percentage game, she went from 10% crazy to 15, to 20, to 30 to 40, to 50 and suddenly a major leap to 100%. The build up was good but they rushed the landing and then the foreshadowing didn't work because of it. This is more a character development thing than an actual foreshadowing thing.
Now to the actual foreshadowing, it's a balancing act. IF you foreshadow something, it needs to be in the readers/viewers mind when it actually happens otherwise it will feel out of the blue. You will need small reminders (a small percent increase towards the goal) between the first foreshadowing (1%) and the pay-off(100%) something they started in the example you mentioned, they just didn't follow through properly.
This, however, has a risky downside. Those reminders will stack up soon and a perceptive person will see the big twist coming.
A generic example: In a zombie apocalypse setting you can have people talk about a weak spot in a wall that needs to be patched, but keeps getting delayed and ignored - then, boom, in a crisis situation the weak spot gets exploited and people die. You can do this in chapter 5, but when the wall breaks in chapter 10 most people would have forgotten this minor detail. If you keep mentioning it for 5 whole chapters the reader will know it is going to break sooner or later making more of a waiting till the clock runs out situation than actual foreshadowing.
If your goal is to write your specific example, the solution would be to make the last two seasons longer and make her gradually become more unhinged because the big problem was simply they wanted to do to much in to short of a time span.
Else generally speaking there is no magic number that fits all scenarios. There is a big difference between the foreshadowing of events and character development. That's why so many fail at it, either it is so well hidden that people forget about it or so obvious they predict it far before it happens.
A good example (Sixth Sense spoilers)
Would be the Sixth Sense. Everybody knows the big plot twist at the end that Bruce Willis his character was dead all along, but the foreshadowing was done during the entire movie by subtle nods that nobody except the boy ever acknowledged his existence.
