At what amount is digesting soap unhealthy? Several of my friends, when they do the dishes manually, do not rinse the dishes after cleaning but merely dip them in the dishwater and then dry them (using a towel or a dish rack). I myself on the other hand always rinse the dishes with fresh (hot) water from the tap before drying them, mostly because the idea of having a dose of soap with my next meal freaks me out.

But is there some guideline as to how much of the soap you can ingest until something goes wrong, be it the taste of the next meal or some poisonous reaction? The soap used for doing the dishes manually doesn't have any warning labels on it... I've found this describing poisonous effects of automatic dishwater soap, and this pertaining to a dish soap that is used for manual dishwashing.
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.