You are surely more advanced than me, but here is my two cents. I sit down and practice in half hour chunks and pay very close attention to what I am doing. My max is 45 minutes. At that point, I am mentally exhausted and my attention is gone, and i take a 15 to 30 minute break. Two hours a day has been my limit. I guess I have not usually been so specific in my goals as others here. Sometimes, l will say to myself, I am going to learn this line of music, or fix my timing here, or learn this particukar scale or practice 3 against 2, etc. But mostly it is more casual, whatever I am doing, I arrive to the piano fresh, and take breaks. I do get frustrated about daily progress too, but when I look at where I was a year ago, my, I have learned a lot! Maybe you need to look back once in awhile and give yourself some congratulations.