Bitcoin difficulty is retargeting every 2016 blocks, which is about two weeks. You could also expect every 13 days the difficulty is increased by 7% percent (thus you mine 7% less coins everyday after this), but the difficulty sometimes go down, sometimes goes up but the long term trend is upward so far. Check https://bitcoinwisdom.com/bitcoin/difficulty
to get the graphs of past difficulty changes. The upward long term trend is clearly visible, thats why you should put in mining calculators +5% to +10% difficulty increase after every two week retarget period to get better idea how many Bitcoins you can mine with your S9.