If you truly want to manage your time effectively, stop thinking in terms of amount and start thinking in terms of priority.
Divide priorities into musts and wants. Ensure that you have ample time for the musts, then fill in the wants as needed, and ignore everything that isn’t on the priority list—unless you have still more time left over.
If not doing your wants creates problems, they will become priorities. If not doing them doesn’t matter, they will disappear.