Like everything else, it depends. There’s a guideline for star points called the Rule of 500, which is 500 divided by the focal length equals the shutter speed. Wide lenses means fewer star trails are visible at longer speeds, tele lenses will show star trails much faster. So with my 14mm on a full frame I can hold open the shutter about 35 seconds. I like to be lazy and keep it at about 30 seconds through, so I don’t have to dig out my cable release. It’s also a pretty fast 2.8 lens, which means I can push it to about ISO 4000 but stars will be limited, I like to run about 6400 if I can, depending on sky glow.
Now, when I shoot with the 5Dii I try not to push it past ISO 1600, and even that’s pushing it for me. I really use that camera to stack and commit to star trails for the sky these days, or accept there is going to be some motion to the sky. But, by mixing techniques such as light painting and stacking, the results can be worth your time. This is about 20 five minute shots stacked together:
But again, the first answer is “it depends.” A lot of it depends on what you’re going to do with these images. Looking at them on a screen you’re going to be a lot more critical, iPad or 19" monitor, you’ll see all the little noise flaws, you’ll hate it, especially at 100%. But once you print it, even at 11 x 17 you’ll have a lot more forgiveness for yourself, you’ll see the noise a lot less, you’ll accept the piece with its flaws more readily, the way you accepted that 4x6 print from your point and shoot.
So don’t get too wrapped up in grain if your goal is to print. You might find out that it’s more helpful than you think. As a friend of mine says “don’t fear the noise, accept it, work with it.”