Nail art is easy to pick up Emma, all you need is a bit of imagination. I'd say a day course is sufficient, someone to teach you the basics then you take it from there, practice and experiment. Rachel from Ka-Sa-Ra aka Chocolate has been offering free nail art courses but I think she's fully booked but has said she'll do more later.
Airbrush courses are also 1 day, you learn the basics then go away and practice. Most places that offer courses will usually offer advanced courses too.
You don't need to be artistic either, just have an imagination. It can be fun!
I lurvvvvv nail art it is one of my fav treatments you can have a look at my albums on my web site there is some of my hand painted nail art and some of Mui's fab nail art and some from japan in the friends album.