This all happened last century. I applied for an "L" clearance after working for 8 years in commercial facilities. I had previously held a clearance in the Navy, DOD. I had kept complete and accurate records. It took ~3-4 months for the approval. I don't know if the DOE accepted the DOD info. If you previously had a clearance with the DOE, they will only need to update since it lapsed. If not, they may want to go all the way back to your childhood to verify the data. I believe that the DOE check is more extensive than a commercial check (you may have access to government secrets). If your records are complete and accurate, your references are easily available, and the agencies are quick to respond, the process will go faster. If you have had any adverse events in your life, or they discover something that you didn't mention, it will take a lot longer. I've worked with some people who said that they were hired and sat around for months on 40 hrs/wk doing non-clearance busy work.