The major variables in the reaction are the temperature of incubation and the composition of the reaction buffer. Most companies supply 10x concentrates of these buffers with the enzymes. One unit of enzyme is usually defined as the amount of enzyme required to digest 1 ug of DNA to completion in 1 hour in the recommended buffer and temperature. In general, digestion for longer periods of time or with excess enzyme does not cause problems unless there is contamination with nucleases. Such contamination is minimal in commercial enzyme preparations.
Preparative reactions contain more DNA in a larger volume.
Approximately 10ng is visible in a single band on a horizontal agarose gel. The maximum volume of the digest depends on the size of the well in the gel. Most wells hold 10-20 ul. When calculating the amount of each component to add, make sure to add extra to allow for pipetting error. For examples, if you are preparing a master mix to digest 6 samples, add 7x of each component.