I've been thinking of how 'yet' in the affirmative statement works. I think [1] through [3] work but I'm not sure about [4].
[1] A: We've got to hurry!
B: Calm down. It's early yet.
[2] A: Is he coming back soon? Has he finished his job in Beijing?
B: No, he has two years yet.
[3] We have a long time yet. We can go to the gym.
[4] We are thirty minutes into the work yet. But I'm already tired.
Best Answer
Yet is generally used in questions and negative sentences. It does not work in the affirmative sentences which you have posted above. In formal style, yet can be used in affirmative sentences like so:
You can find more information about te difference between still, already and yet in the Cambridge Online Dictionary.
In your sentences, you should use still or already. Already is used for things that have finished and still is used for things that are still going on at the moment of speaking.
Your sentences would be correct like so:
In your last sentence, I wouldn't use still or already, but I would use only to contrast the already in the second sentence.