I was late starting my practice on Duolingo one day and got a reminder email so I assume if Duolingo sees you're active enough in the last 24 hours it does not send a reminder unnecessarily that same day.
Interesting, I have unchecked then rechecked it and I still haven't received an email. I've also left it checked for a while now and haven't found any emails. I did change the time to 8pm but had left it at 5pm. Nothing seems to have worked for me.