Alfa recomennend changing it every 90k.
But the waterpump is the weak spot and its not uncommon for them to sieze and take the belt of, chances of this increase at over 50k so most specialists recomend changing it at no later than 60k.
There is no official Alfa recomended change inteval of the water pump but it needs to be done at the belt change, along with the idler and tensioner pulleys which will come in the belt kit.
Its also worth mentioning that the Alfa dealers book time for changing the belt is about 8 hours labour and so is a £1k job as it includes taking the engine out..
This is not necessary so most specialists charge about £400.