Nice to have another member from Switzerland in the forum!! (I'm not really FROM Switzerland; I'm from California. But I've lived here for over 15 years -- near Basel.)
PS -- I'm a part-time English teacher and your English is REALLY good!!!!
I don't have any experience with Kingsnakes, but like Merlin and Venus said, how long you can/should hold them depends mostly on your snake. You will be able to tell if the snake is not comfortable or if it's had enough.
My Ball Python loves being held and going for a little exploration trip across the sofa or carpet. I take him out almost every evening if he's not digesting or in shed.
My Rainbow Boas are not as tame, though, I can only take them out in the evenings and only for about 10-15 minutes before she starts getting agitated. They only get taken out a few times a week.
It sounds like you're doing the right thing. As long as you leave the snake alone for a few days after eating, you should be fine.
As far as feeding: you can feed 1-2 mice that are the same diameter as the thickest part of the snake's body.
Check out the Reptilien Center in Schlieren:
http://www.profiterr.ch/
They have various types of corn snakes and a few months ago they had quite a few baby ball pythons: as well as almost any other reptile, scorpion or spider you might want. The owner, Martin Schreiber, is very nice and VERY knowledgeable. The animals there might cost more than at a pet store but they are well-kept and healthy.
Tschüss!
