You can pick from four levels of compression.
The level of compression attained depends on a number of factors, such as how much the file is already compressed. For example, JPG files often compress very little. The level exact percentage of compression cannot be accurately predicted by any compression program.
As a general rule of thumb, HIGH compression takes a lot of extra time for a little bit more compression. MEDIUM is the default.
LOW is quicker, NONE ends up with a file slightly larger due to header information added.
Overall "normal" business data compresses to about 50-60%. Some files compress to 90%.