In practice this isn't very common. In most cases if you block the email that they got a hold of they will just give up rather than trying random addresses.

However it does happen a little. Because it sounded like fun I actually added a signature to my addresses. So it looks like github.com-abcdef@example. Then the spam filter will check the signature and if it is invalid it will give it a high spam weight. (Other than a few widely-published addresses that are accepted at neutral weight)