As a landscape photographer, you feel the need to get out there and be creative, stretch the jaws of time instead of freeze a moment perhaps. Very long exposure is the order of the day (or I should say night perhaps). Here is a tip on how to get things rolling. Let's say you put a ND graduated filter in front of the lens to balance the exposure between sky and land, 3 stops soft. Then you do a test with the lens at f/22 (never mind the diffraction, you are after a soft dreamy image) and the camera registers a 1 second exposure.
Now you add a 10 stop ND filter in front of the lens, hence you extend the exposure time by another 10 stop, in which case the 1 second exposure will become 2 > 4 > 8 > 16 > 32 > 64 > 128 > 256 > 512 > 1024 seconds. So, if you leave the shutter open for 1024 seconds, you will get a perfect exposure, or will you?
During the film days there was the term 'Reciprocity Failure'. This meant that if the recommended exposure by the film manufacturer was up to 5 seconds long and you wanted to expose the film for 10 seconds, you should take into account the film maker’s instructions on 'Reciprocity Failure'. Digital is similar but on a far smaller scale.
Now see this. If you took the picture with 1024 sec exposure, you would probably be fine. If on the other hand, you left the camera exposing for another 500 seconds, you would only be 1/2 stop overexposed, something very easily sorted in software. Besides, we need to remember that underexposing or properly exposing a scene (talking about long exposures) produces colour noise and other artefacts. It is always best to overexposure a scene by at least 1/5 stop.
My point is, next time you shoot a scene with recommended exposure of 3 minutes, let the camera expose for another 1.5 minute. If the final scene is a bit overexposed, it can be fixed in software (assuming you shoot RAW). If the exposure is fine then bingo. Remember one more thing. Slightly overexposed images in long exposures, have much less noise in dark areas than properly exposed ones.
In fewer words and you may want to write this on a bit of paper and add it in your backpack.
Using a 10 stop ND filter:
If your standard exposure is 1/4 second, go for 6 minutes.
If your standard exposure is 1/2 second, go for 13 minutes.
If your standard exposure is 1 second, go for 28 minutes.
If your exposure is longer than 1 second, do the maths yourself, write it down and keep it handy.
These are rough guidelines. Exposure times vary with individual filters. From experience you will realise what is best for your own camera.