Official Fiat answer is 72000 miles or 5 years for the 1.2 8v engine, 72000 or 8 years for the 1.2 16v engine and 72000 or 5 years for the diesel engine.
You should change the belt at at the mileage interval or the time stated which ever comes first.
The 1.2 8v engine is a "safe" non interference engine so if the belt goes the pistons do not hit the valves, so you can drop a new belt on retime the engine and off you go. Although this is would be very inconvenient and possible dangerous if the belt failed on a fast road or motorway. The 1.2 16v engine is an interference engine so if the belt breaks you will bend valves and possibly damage the pistons resulting in a very expensive repair bill.
The 1.2 8v engine is really easy to change the timing belt on and an experienced mechanic could do it in under an hour, the 1.2 16v is more complex so likely to take a little longer. Can't comment on cost as I do my own so just the cost of the parts which is about £40.
