I've worked on Fiat engines that had pretty much the same configuration as the one you describe.
Once initially set, they never needed readjustment, as others have mentioned, unless you change something like grinding valves, replacing valves, etc.
On one particular Fiat 128 that I worked on, the owner had just had a valve job done at a dealership in Houston. The car was running pretty poorly, and I found the valves badly burned. The 'dealership' mechanic had ground the valves but had just reassembled it... he never reset the clearances by using the disks, the valves went bad immediately, and the dealership wouldn't even respond to the owner.
I re-did the work, set the clearances (in one case had to grind a bit off the end of the valvestem), and put it back together. The engine lasted until the car collapsed into a pile of rust, considerably later.
So if you ever have to have valve work, be sure that the person who does it knows what it is all about.