Let it sit for 20–30 minutes. Do not flush. Do not lean over the bowl. You may hear fizzing or feel heat radiating from the porcelain.
If your blockage is physical (a toy car, a broken toothbrush, or tree roots), caustic soda will do nothing but sit on top of the problem, creating a dangerous puddle.
October 26, 2023 Subject: Evaluation of Caustic Soda (Sodium Hydroxide) as a Drain Cleaning Agent for Toilets