A320/330/340
As mentioned in a previous post the A320 type rating covers the A319/320/321 and soon the A318 - most companies hold a short differences course for each variant.
The A330 and the A340 are separate type ratings. It is possible if you have sufficient experience on one of the airbus types to do a shortened type rating conversion course called a CCQ (cross crew qualification) this course lasts between 7 to 10 days depending on types - for example the A320 to A330 or vica versa is about 7 days - the A320 to A340 is about 10 days. A full conversion course on each type is about 5 weeks.