It all depends on how dark it is. In a truly dark sky site you will probably need 20 secs, wide open or f4 at ISO6400. If there's a moon, things change a little, and such speeds and ISO's will result in over exposure. There's no one size fits all.
As a rough guide...
True dark sky site with no moon - 20 secs, ISO6400 f4, (or any other equivalent EV)
Dark sky site with half moon - 20 secs ISO1600 f5.6 (or any other EV equivalent)
Dark sky site full moon - 20 secs ISO 800 f5.6 (or any EV equivalent).
Add light pollution to the equation and things get more complicated... and trial and error comes into play.
Basically though... with most wide lenses, you have 20 seconds to play with until trailing starts. The longer the lens, the shorter that time gets. With a truly wide lens like a 14mm (or crop equivalent) you may get 30 secs.