After setting a timer on your iPhone, you can either glance at the Lock Screen or open the Clock app to see the remainder before it rings. Sadly, there’s no in-between; your iPhone either needs to be locked or you’ll have to glue your eyes to the Clock app.
That’s something that a new and free jailbreak tweak called StatusBarTimer by iOS developer ldefix2020 aims to fix, as it puts the timer remainder in the Status Bar so that you can see it from just about anywhere in iOS.