The instructor at one of the other sites I follow (Learn and Master Guitar) suggests (for a 30 minute session):

5 minutes of warm up
10 minutes of practice. This is dedicated to whatever one may be working on (scales, chords, soloing, etc.)
10 minutes of learning a song.
5 minutes of improvising or experimenting. This is just doodling, I call it putzing.

I have tried to use this and have a spreadsheet to log my practice. I try to practice an hour at a time and have modified it to an hour. Then I try and practice an hour in the morning and an hour at night.

I figure you don't need more than 5 minutes to warm up. And I will skew the practice a little depending on whether I need more work on the practice part or if I am working on a song and need a little more there.

5 minutes warm up.
15 -20 minutes of practice
15-20 minutes of song learning
5 -10 minutes of "putzing".

It's just what works for me.