There are a lot of conflicting views on this topic. You need to form your own opinion based on what works best for you. From 1990 to 1993, I was studying with George VanEps. He taught me his warm up exercise which took about 20 minutes to execute. He mentioned that he had done this every day before practicing for 30 years! I quickly added up in my mind that he had played this simple exercise over and over again for a total of 3,630 hours. I decided that instead, I would spend 3.360 hours learning new tunes, advancing my technique, promoting my career or learning ANYTHING  I didn't already know!

 Also, how long is your attention span? (For most humans, it is around 20 minutes!) When is your mind most receptive to new ideas? (Probably in the first 20 minutes of your practice time.) Is it a good idea to play something you already know, which needs no thought, over and over, wasting what could be the most productive time of your practice session?

When I taught at the Wisconsin Conservatory of Music, I would teach students for the four years needed for a bachelor's degree. I noticed that after being with me for a year, one student hardly improved at all. One day I asked how he practiced. He said that he did a warm up

every day before "practicing": every scale, in every key, in every position! This took about an hour and a half. I said what do you practice after that? He said, he was too worn out physically and mentally to accomplish much of anything after that.

For me, it works best if I play the items that require most thought, FIRST!  I'm just careful not to overtax my hands before they are ready. Remember, your hands don't know the difference between a boring exercise, playing interesting music, or brushing your teeth, but your mind does. I would rather receive the enjoyment of playing exciting music and the challenge of learning.
   

Your hands are busy all day whether they play music or not. You should try not to hurt your hands whether you are playing the guitar or cutting the grass. However, when playing the guitar, you need to remember that your mind and heart are where the music comes from. Don't waste your time daydreaming while playing the guitar. You need to be thinking about what you are doing when playing the guitar.

There will come a time when you will no longer by able to play and your career will come to an end.  Do you want to look at your past life and see that you've spent a million hours playing meaningless warmup exercises? 

Summary:  If your attention span for peak constructive thought is 20 minutes... if the Mayo Clinic suggests only working the hands 20 minutes separated by rest periods... Conclusion: You can practice most efficiently without hurting your hands by practicing numerous 20 minute sessions instead of one long one.

The bottom line: How much did you get done in your practice session without hurting yourself?