For owners, there are two very important Assembly Numbers in the history of the 65 deg V12s with twin rubber cam timing belts. The first of these is Assembly Number 28614 for the 456 and possibly the early 550. At this Assembly Number Ferrari updated the cambelt tensioner pulley bracket to a newer model. This tensioner bracket remained unchanged throughout production of 65 degree V12 cambelt engines and through the 612. Ferrari Parts : 171060 : 171060 PULLEY BRACKET
The second important Assembly Number is 43847, very early in 575M production (all US cars were later and the Geneva Autoshow car was Assembly Number 43148), when an improved cambelt was introduced that allowed cambelt change interval to be changed to 5 years or 50,000 km (~30,000 miles). When this belt was introduced in late 2001, it superceded the old belts for the 456, 456M, 550, and a very few Euro spec 575Ms.
The point of all this is that all of the 65 deg V12s with a recent belt change have the latest belts. If owners of early 456s (and possibly early 550s) change out their tensioner pulley brackets, they will have exactly the same cambelt hardware as the 575M and 612, which have a five year belt change interval. From there, draw your own conclusion on what belt change interval should be.
Now that we have looked at a little history of belt hardware, time for a bit of history on belt tension numbers. For the 456, 456M, and 550, the recommended tool was the Staeger FST 50 Tensiometer. This tensiometer measured the acual tension of the belt between the pulleys using a depression of the span to calculate tension. Results of this type of tensiometer were not particularly precise and not very repeatable.
For the 575M, a new type of tensiometer was recommended that used the harmonic frequency of the belt span to measure tension. This method of measuring belt tension proved to be very precise and repeatable. The recommended tensiometer was a Seems Tensiometer Type Diapaz, which cost around $2K. The nearest equivalent was the Gates 507C Sonic Tension Meter, which retailed for $800 and could be occasionally found for a bit less. Both of these tensiometers were way more complicated than necessary since they were designed for multiple applications, and not just automotive belt use.
Luckily, the Ducati folks found a much simpler tensiometer just for automotive or motorcycle belt applications, the MotoReva MT-112, that only costs $249.99. MotoReva - Ducati Tools and Supplies
So now down to the real meat of this thread for anyone who would like to do his own belt change on one of the 65 deg V12s. These V12s all use the same belts and the same tensioner hardware, so the harmonic frequency for all the cambelts is the same and we know what that is from both the 575M and the 612 workshop manuals. From the 575M WSM, the frequency when the longest span is struck is 126 hz. The 612 WSM expressed it differently and wanted measurements of both the spans on one cambelt added together and the sum of those frequencies should be 335-360 hz. Logically, the 612 frequency was more than twice the frequency of the 575M WSM single span because the shorter cambelt span has a much higher frequency.
Note that this tensiometer can also be used to measure the tension of the auxiliary belts, and the 575M numbers should suffice for the 550, too, but I am unsure of the 456 and 456M applicability. 612 owners have numbers in their WSMs.
Here are the applicable pages from the 575M and 612 WSMs.
Any pros jump in here if I have gotten any of this wrong.