A good practice routine should have a good mix of theory, chords and scales.
In the beginning you'll be playing a lot of repetitive exercises to build muscle memory. These exercises can be dull so you also need to have some fun stuff.
I would suggest using the chord changes exercices and scales exercises you can find in the beginner's guide as a building block for you routine.
A simple practice session might look like this :
Theory - 10 minutes (or more if your are working on a long tutorial)
warmup - 5 minutes
Exercises - 20 minutes (chords changes, scales exercises)
Break - 5 minutes
Fun stuff - 20 minutes (learning a song that includes the chords you've learned)
I've used this format for a long time and it proved to be the most effective for me AND the one that gave me the most fun. Sometimes you may need to adjust it if you are working on a tutorial that requires more attention. But it is a good building block.
As far as how much time you spend on a perticular exercice? I would say until what you play feels comfortable (ie, you don't feel pain or fatigue once you're done), that your execution is good (ie, you can play the exercise cleanly) that you have reached a speed you like. Personnaly, good execution will always be more important than great speed. So if you can play something perfectly and lower speed, you can say you've mastered the exercise. Speed is an whole other beast. It comes with time.