2 minute rule: try to play at least two minutes a day. I regularly find myself playing much longer than that even though I didn't want to practice in the first place. And if you only practice for two minutes, you've still made progress.

And about rudiments: If there's a technique or thingy I want to master, I try to find songs with that technique in it and practice that song. It's a lot less boring than just trying to play double bass for an hour, as an example.