Just to be sure I reviewed my collection and agree with Carl that it is likely most SLR's have bulb mode.

How long do you hold it open for will of course depend on the scene lighting, film speed & reciprocity, aperture and the effect you are trying to achieve. You can estimate the exposure time using your camera's built-in meter or external meter. Or, some cameras can auto expose very long exposure settings.

In the example image below, I used the Pentax LX's aperture priority auto exposure to "correctly" meter the scene. I had no idea what duration was required to smooth the ocean waves so I "bracketed" by taking the same scene as it got darker in the evening - giving longer exposure times. I could have used ND filters to achieve that too.

Link to larger image version http://www.fototime.com/773634514E5AF6D/orig.jpg