[bentrevor]

You can use `osascript` to show a popup. Depending on which shell you're using, it would look something like this:

    for i in {1..4}; do
      sleep $(expr 60 \* 30)
      osascript -e 'tell app "System Events" to display dialog "Are you wasting time?"'
    done

[cweagans]

Or just put the osascript line in your crontab?

[swsieber]

You could do notifications via hammerspoon. It's probably overkill, but you could probably work in a custom active window time monitoring system.