How long to spend at carlsbad caverns? -

My boyfriend and I are in great shape and love to hike. We are going to Carlsbad Caverns in late Dec/early Jan and want to know how long we should plan on spending. We don t plan on booking a guided tour, just the self-guided. Are the Natural Entrance and Big Room tours separate, or is the Natural Entrance tour just an alternative entry besides the elevator but covers the same main terrain? Would it be feasible to do both? How long does each one take, or to do both? Thanks!

I found that 3 or 4 hours was enough to see all of it.The lower caverns require advance reservation and are quite limited.