|MadSci Network: Chemistry|
Mayonnaise is a suspension of oil droplets in a water-based continuous phase (vinegar), stabilised by molecules found in egg yolk that have both an oil-soluble and a water-soluble end.
The size of the oil droplets and the concentration of the stabilising molecules on the surface will determine how long they can stay dispersed before separating out of solution - if they are small enough, they will stay dispersed for longer than the natural lifetime of the mayonnaise. (I don't want to think about what happens next...). Larger droplets, on the other hand, will glob together more rapidly. The more energy you put into dispersing your oil droplets in the first place, the smaller the droplets will be.
I guess the answer to your question is that most mayonnaise will contain a distribution of oil droplet sizes, with some droplets less than a millionth of a metre across (so obviously of colloidal dimensions) and others much bigger. So it will be a suspension that will be a colloid to a greater or lesser degree. If it is a store-bought mayonnaise prepared with an industrial homogeniser, the droplets will tend to be smaller, and probably won't coagulate much before the mayonnaise goes off; if it is one you've made yourself, there are going to be a lot more large droplets, and these should start to coagulate and separate out while you watch.
I could talk for hours about emulsions and how they are stabilised/destabilised (and I have: First Hour, Second Hour)... but with any luck this will be sufficient answer to your question!
Try the links in the MadSci Library for more information on Chemistry.