Twice a year, but when you have done it less than 6 months ago and the car was protected after that time all the time then once a year is good too. To feel if your car needs claying or not is easy to find out with a plastic bag: put your hand inside a clean sandwich plastic bag and feel the the paint. When it need to be clayed it feels bumpy and rough, do the bag test on that spot again after you've clayed it and feel the difference.
On a car which is never clayed it takes longer, because it is dirt on top of dirt on top of... etc and you can not rush it. The first time on our Jazz (which was 5 years old then) took about an hour. Now i do it in half an hour.
I use Dodo Juice Supernatural Fine Detailing clay with their clay lube, also Bilt Hamber Soft Clay is a good choice. But there are many more, only keep in mind that the Jazz has soft paint.
There's lot of information on car detailing fora.