Hey MS2013,
To add to ViceVersa’s information, there are a number of ways to decrease lightmap build times, but at a certain point it comes down to your computers hardware (graphics card, ram, and processor), as well as the density and scale of what you are trying to bake with Lightmass.
One location to look is the ‘World Settings’ tab. This tab contains a section which you can expand called ‘Lightmass.’ Here there are various options to choose to change the way your Lightmass calculates and accounts for different variables. Take a look at this section and hover your mouse over the options to read the tooltip for information on what the settings does and what it effects. Another way to decrease lightmass build times is to reduce the overall lighting quality. If you are constantly making changes and have short iteration times when baking your lights, set the quality to something like ‘Preview’ or ‘Medium’, just to get the general feel and point across. Switch back to ‘Production’ for your final lightmass build.
Below is a Lighting Troubleshooting Guide which contains A LOT of helpful information, including the settings mentioned within the ‘World Settings’ tab.
Lighting Troubleshooting Guide
Cheers,