Don't know if this helps or not, but I took my 2006 2.4JTD in for some repairs this morning. I have an "Autoguard Warranty" that is valid for the lifetime of the vehicle as long as it is serviced "In accordance with the manufacturer's recommendations."
When I contacted Autoguard to claim, they told me that although my last service interval was 10000 miles or so, because the time period between services was 15 months it invalidated the warranty as this was outside of the manufacturer's recommendations and also the info supplied to them by ICME. They therefore cancelled the policy and refused to pay the claim.
However, seeing the confusion in this thread, I contacted Fiat directly - not a dealer - and got the following reply:
"Further to your request, we would like to confirm that we recommend that the vehicle must be serviced in accordance with the frequency and content detailed in the appropriate section of the Owner's Manual. We also recommend that all servicing should be carried out by an Alfa Romeo appointed dealer or by an independent VAT registered service/repairer outlet.
Your Owner's Manual will establish that a service must be carried out at every 18.000 miles; however, some items have a maximum time requirement; for instance, the brake fluid and polen filter must be changed every 24 months, as well as the engine oil and oil filter (unless your onboard computer advises you to have the oil and filter changed via a message or warning light).
Should prevailing use of the car be under specially heavy conditions, such as trailer or caravan towing, driving on dusty roads, driving short distances (less than 7-8 km) and repeated with external temperatures below zero, frequently idling engines or long distance low speed driving (e.g.: door-to-door deliveries) or in case of a long term inactivity, or driving on urban routes, we also advised to have a few checks carried out more frequently than required on Service Schedule."
As I say, this came directly from Fiat/Alfa Romeo. I forwarded this to Autoguard, who have accepted it, changed their database, renewed my policy and are paying the claim.
I am not making a comment on how often you should get it done, merely stating what Fiat have recommended to me so if there are any garages out there trying to make people have the 12 month service then they are more than likely trying to rip you off (or they are just as confused as everybody else!!).