In the beginning three years ago it has taken me around 3 days to sketch the graphics for my wallpapers on paper. Then i took a day and tried to copy it in Photoshop. But it rarely came out as i wanted it to be.
In the end of last year i bought a Wacom tablet and made my first steps with it. Working with this tablet makes things more dynamic and faster. I took around 3 hours for this. From the beginning to the end. I have a little library of colors i take from photography i do so it´s easier to choose a color palette for a wallpaper. I hope this helps. If you need more help or information feel free to write a note or mail.