I'm in a bit of a scale/chord/arpeggio drill phase right now. This stuff is everywhere in music, but also useful in cultivating good practice habits like having clear goals, careful listening and kinesthetic ease. Just imagine being able to play them beautifully at any time, I always think of some of my favorite pieces where there might be a quick scale run that sounds so incredible and imagine the day when I can come across that section in music and be able to play it right off the bat.
Anyways, a few thoughts:
Don't worry about playing for 20 minutes or any set amount of time, just get yourself motivated and practice until you reach your goal (or if your initial goal is too much, just adapt midway to whatever you can accomplish), or until you get mentally exhausted because to push too far past mental exhaution is asking to develop bad habits and build resentment. Like Debussy, I often get into it and by the time I'm done an hour has passed. I know I'm making progress if I've lsot track of time!
Let your weakness decide your goals. If you have troubles with a certain speed, let that be the goal. If you can achieve speed but are inconsistant, aim for consistency (like not leaving until you can play it accurately 2X, 3X, 4X). After a point your scale practice becomes less about the technique and more about mental focus and listening. Concentration is perhaps the most useful skill to develop.
Build on what you can already do. If you can already play a scale consistantly, it's time to do staccato, crescendos/decrescendos, pianissimo, fortissimo. I find staccato is more about the liftoff than how I press the key.
Because it can be boring, I sometimes find myself nodding off. Just yesterday I found the solution for this, I just let myself lay back and have a little snooze right at the piano! Hell I must be tired anyways, and I can't long at the piano like that so it's just a few minutes dozing... Then I wake up and all the tiredness is out of my system and I'm ready to rock!
A difficulty I've found is to be able to do something right off the bat. What I mean is being able to sit down and play the scale cleanly right away. After practicing arpeggios for 10 minutes I can do it no problem, but then if I suddenly try playing a scale I can't do it because my fingers want to automatically sprawl for an arpeggio... So that's something specific I have to practice as well - changing gears.