I had a large LMB (~12-13") die on me a few months ago, I left around 8 am, got back around 5 pm and the smell already had my room smelling awful. Since he was solo in that tank, I broke everything down and took it outside, but the room still stunk. I opened the window, ran two fans and by the morning it was alright. I can only imagine that after 5 days, it must smell right nasty. The suggestions here are all good but get those windows open if you can, blow the fans and clean that tank good, big water change. Hope the GF survived, haha.