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Juding long exposures can be fairly hit and miss. There are some strategies involved. I'll discuss a few things...
First of all, f/25 is a *really* small aperture for using on a DSLR. You'll start hitting the diffraction limit at around f/16 and losing sharpness as a result. I usually keep my photos at f/11 or wider, only occasionally using f/16. Something like f/25 will require a really long exposure time. This photo was at f/9 and I still needed 30 seconds at ISO 400. If I had been at f/25, which is 3 stops slower, I would have required 240 seconds (2^3)*30 to get the same exposure.
The other thing is, you often need to use higher ISO. As I said, I used ISO 400. If I had stuck with 100, the exposure time would have to be 4 times long, or 120 seconds.
If I had used ISO 100 AND f/25, well, then we're looking at a 960 second exposure to get the same image, which is quite a long time to wait. Especially if you don't get the exposure right.
So yeah, basically you definitely need to open up some more, and possibly turn up the ISO depending on the conditions. One trick I use to judge exposure time is turn the ISO all the way up, and then set the aperture on the camera wide open and get the meter reading. Then I do all the f-number calculations to determine the shutter speed for my desired aperture and ISO while still getting a correct exposure.
I'm not familiar with Nikon gear, but if you plan to use bulb mode you should certainly try to find a timer remote like the Canon TC-80N3 (not sure what the Nikon equivalent would be there). I've found it to be a big help because I can dial in the precise exposure time, set up my scene, click start, and then just sit back and relax while it exposes.
Couple of other hints, for long exposures, put something over the viewfinder. Light can leak in from the back of the camera and show up in your image if your exposure is really long. A piece of cloth or a hat usually works well enough, just make sure to trigger remotely to avoid shake. A stable tripod is also highly recommended, especially if you plan to shoot somewhere windy.
Hope that helps, if you have any other questions feel free to ask.