There are a variety of factors that influence this:
- The size and breed of dog will have an impact on how much of a toxic substance a dog needs to ingest before the effects become visible.
- In many cases, whether or not the toxin is ingested on an empty stomach will also be significant. (So, a small amount of a toxin present in a large quantity of food will usually pose less of a threat than the same amount of toxin ingested alone.)
- Frequency of exposure can also play a big role. Many toxic compounds will stay in the system for some time before they are broken down and/or passed. Others will cause damage that may not be immediately symptomatic but can become so if the body is not given time to repair itself between exposures.
For chocolate/cocoa, it looks like 1.3g/kg of baker's chocolate will generally cause symptoms. (eg. a dog weighing 10kg would need to ingest at least 13g of baker's chocolate for symptoms to present.)
For grapes and raisins, even 10g/kg can be problematic.
For onions, 15g/kg is enough for some dogs to show signs of the toxin. (The amount for cats is even less: 5g/kg.)
Also, since you specifically asked about pizza crusts:
All forms of onion can be a problem including: dehydrated, raw or
cooked onions, table scraps containing cooked onions or garlic,
left over pizzas, chinese dishes, any feeding stuff containing onions.
- Some Food Toxic to Pets
The quantity of macadamia nuts required to cause symptoms seems to vary widely. Apparently as little as 2.4g/kg can be enough to induce symptoms.