It depends on the light technology. Too much incandescent or HPS can cook your plants. Fluorescent can be as close as possible short of touching. LED's are presumed safe but I don't have personal experience using them as I am genetically too much of a cheapskate.
Basically, plants need light like they need water. Either too much or too little will harm or kill them. The term DLI (daily light integral) is the best method of calculating optimum amounts of light for plants.
Attached is a Word doc, spreadsheet and PowerPoint that explains as much of this as you can bear
