it should tell you all this info in the handbook of the car,if you dont have one try to buy one from citroen or even try ebay,as it will come in very handy in the future,for lots of things,or just phone you local citroen garage and ask.
For the price of a belt to what is would cost you if the belt snaps is hundreds, the belt should be changed at least 50.000, whilst you are getting that done replace the fan belt, power steering belts the same time, Costly??? It makes sense. the job is then done once.