In reply to mwr72:
How long? I have usually found somewhere between 10 seconds and 1 minute works well. Things like clouds go almost too quickly at the longer end.
I find the biggest problem is power. My d7k keels over after a few hundred shots even with a battery grip in daytime. Things are far worse at night when you have long exposures to contend with.
The kit I used for filming in norway last year was:
D7000 with battery grip + rain cover.
Cheapo timer with rubber band to connect to tripod.
Big box of AA batteries. The grips will take rechargeable camera batteries but you need loads of them unless you have easy access to power.
Fairly basic tripod, with a length of accessory cord tried to the bottom of the shaft. This was generally tied to a rock or tent peg directly under the camera and tensioned.
Oh yeah, and set the camera to manual mode. I accidentally left mine on aperture priority and ended up with an evil flicker that I have to somehow remove before I can show anyone my stuff. Stupid.