The stars move for one degree in four minutes. So if you want to capture them still, you don't want to use big exposure time. The exposure time you can use actually depends on your shooting settings.
The bigger crop factor means the smaller exposure time you can use for shooting still stars
If you are using a zoom lens you might also want to use smaller exposure time to avoid the stars getting blurry
To capture some beautiful star trails try to get as far as possible from light pollution of the big city. You might also want to check the weather report to make sure that there will be any stars visible on the sky.
Select the lengths of the tracks you want to get on your final photo
Get a live preview of the result
The timer has also a frames counter, which is handy when you are going to make more than few frames