[scarcely]

You could look into this app called Sprintwork. I set the profile to be fixed for thirty days and so far I've yet to find a way to circumvent it.

If on iOS, the parental restrictions are awesome.

If on Android, restrictions are worthless so I usually use a blocker, set the phone to unlock by fingerprint and in addition have a system password (because of the possibility of logging in via "safe mode" which I so regret having discovered) written down on a piece of paper and hid inside my neighborhood Starbuck's sofa.