Many of my blog reader will know that I love to work with long exposures. Something about capturing images that have changed in the duration that the exposure took place in fascinates me. We see the individual parts of the scene changing over time but the camera records all of it as a single frame - something that is tough to visualize.
The only way to get the result you are happy with is to experiment with the duration. What may work on one subject matter on a given day may not work on another day or subject. My suggestions are to start with a 1s exposure, then keep doubling it until you are happy with the result.
However, as you will note from previous blogs, you will need to use Neutral Density (ND) filters. They are usually available in 1, 3, 6, 9 and 12 stop ranges (with other ones in the mix depending on manufacturer). Remember, each stop of ND filter allows you to double the exposure time (for a given ISO and f-stop). For example, if your camera meters 1/2 of a second, then a 3-stop filter will allow you to expose for 4 seconds. You can stack ND filters: 2 3-stop filters is the same as a 6-stop filter.
This image was exposed for 20 seconds using a 12 stop filter.