ATTQOTD: I think I've read in various training guidance that a "long run" is something over an hour.
When I'm cheering my connections on Garmin, I don't say it's a "long run" unless it's at least 7 miles. This is sort of to make sure it's over about an hour and so I'm not calling everyone's daily 10k a "long run". If it's something you're doing frequently (like some people do for 10ks), then I don't think it can be considered a "long run".
For me, it's usually my longest run of the week, but even so, I wouldn't call it a "long run" unless it was at least 70 minutes. Since I trail run, I do everything by time, because the distances can be quite different depending on terrain.