For me its soap, I have been here getting on for 7 years and only have 2 bars left having bought only 1 bottle of shower gel since I've been here.
Its not that I dont wash, but in the UK I would buy the contents of closed factories after the auctions had finished. Most of what I bought was industrial, but one time I ended up with mountains of soap its just about to run out. Now I dont think thats bad one or 2 showers a day for nearly seven years.
What your longest lasting consumable?