Complete decomposition can take anywhere from a few weeks to a few months , depending on the tank's biological health. Factors That Affect Dissolvability

Unlike facial tissue, paper towels, or napkins, toilet paper is manufactured using specific fibers and processes to ensure it breaks down in water.

Most standard toilet paper is designed to dissolve in water, which is critical for avoiding clogs in plumbing and septic systems. Frank's Mr. Plumber

So, does toilet paper dissolve? It is an engineered product designed to turn from a strong, soft sheet into harmless sludge.