At what amount is digesting soap unhealthy?
I'm less familiar with safety regulations elsewhere in the world, but in the US, "chemical manufacturers" (which includes soap, sugar, salt, and just about everything else you can imagine) have to produce Materials Safety Data Sheets (MSDS) and make them publicly available.

Out of curiosity, I did some digging for soaps. Proctor and Gamble is a huge consumer producer, including dish detergent. So there are MSDS sheets for soap.

The MSDS gives oral LD50 values. That is, the lethal dose required to kill half the population.

The LD50 for soaps seem to vary by animal, but judging from a few MSDS I pulled up, they're ~2 g/kg or so. (That value was for rats.)

So if I had to guess, given, say a ~70 kg adult, we'd be talking about 140 g of soap, minimum to kill a person.

Now.. that's acute doses. I have no ideal if anyone has established long-term toxicity of soaps, and I can't find anything obvious online.