Ethernet over powerline adapters - how far will the data travel?

I believe (and have made an edit to your post) that you actually mean an Ethernet over powerline bridge. These are a fancy set of circuitry that ultimately act as a very long Ethernet cable.

How far it works is dependent on a few factors that ultimately require a combination of testing and test equipment. The most straightforward way to try it out would be to visit a neighbor and plug it in directly to see if you can get a connection in their flat.

As others have mentioned in comments, it's normal for these devices to be simple bridge type adapters that require a pairing procedure of some sort. Once paired, these devices will only communicate with each paired device, not with unpaired devices that appear. You can look for specific models of adapters that include built-in security, such as these TP-LINK ones.

Referencing this answer - Its supposed to stop at your power meter - I do believe the connection between the mains and power lines is coupled, rather than a physical connection, which allows this. The theoretical range is 300 meters, but practically, this is the length of your power lines, and homeplug is a sensitive to line noise, so it will be less.

Every homeplug adaptor I've bought has had a standard pre-set private network name and adaptors on the same power line with different network names don't talk to each other. I'd strongly recommend resetting these to something, since if nothing else, it helps you troubleshoot. Once this is done, you will not be sharing your connection with a non-paired adaptor in your apartment let alone the whole building.

I'd also add, I never picked up anyone elses network when I ran homeplug, and I'm sure at least one of the ~90 apartments other than mine has at least experimented with it ;)