The above file was being used. I removed it from the piece of content it was being used for. 

Does this mean that it will automatically be deleted at some point? If so, when?

I just ran cron and nothing happened.


The cron job system_cron() will delete temporary files after the max time stipulated by a constant (6 hours)

Those six hours is the reason why temporary file aren't immediately deleted when cron tasks run. If then there are many temporary file to delete, a single run isn't sufficient to delete all the files.