When should one use "etc. (etcetera)"?

One rule I’ve seen more than once, is to list at least three actual examples before using etc.

I often use etc. after I’ve listed a few examples, so that I do not have to list all of the examples.

I will give more examples, if I think it is useful, but sometimes it’s only necessary to give one example, because people will know what you meant and be able to picture in their minds the other examples.

For example I might say to a question about what to serve at a party:

Instead of a sit down meal, you could serve appetizers, etc.

You could serve several different types of sandwiches and chips etc.

You could serve some hors d-oeuvres such as bacon wrapped dates, a baked brie, some dips like hummus or black bean dip or hot Velveeta with Rotel, etc.

I always thought you could use etc. after listing 2 items. Listing everything makes the reader lazy, not the writer.

Use it when you might otherwise write “and other [such] things,” because that’s what et cetera (two words) literally means.

The issue isn’t laziness but economy. Often an exhaustive list is unnecessary and even distracting. If you see “Be sure to bring your notebook, pen, etc.,” you can infer that it’s a reminder to bring your school supplies or class materials.

This is not to say that it isn’t often used lazily. But it isn’t automatically and necessarily a device of laziness.

One example isn’t enough because you need to see what those things have in common. Otherwise you don’t know what category they belong to and therefore what category other such things belong to. A notice that says “Donations welcome. We need items of clothing, etc.” doesn’t tell you whether the other requested items are food, furniture, household supplies, toiletries, or what.

I don’t know what category candles and newspapers belong to, so “etc.” in that context tells me nothing.

I use it all the time, and I confess I really do not know the convention.

I usually list only two items myself.

When you don’t want to go in to the minutiae of the thing which for me is often.

I’m with @DaphneT . It’s up to the reader to comprehend.

According to the Oxford English Dictionary…
(ɛt ˈsɛtərə)
et cetera, etcetera

1.1 As phrase: And the rest, and so forth, and so on (cf. Gr. καὶ τὰ λοιπά, Ger. und so weiter), indicating that the statement refers not only to the things enumerated, but to others which may be inferred from analogy. Occasionally used when the conclusion of a quotation, a current formula of politeness, or the like, is omitted as being well known to the reader.

I use etc when it would be cumbersome to describe a list of items or to clarify a previous description. In the list of items before the etc, it is a good idea to make them as varied as possible to give an idea of the full range of possibilities.