The trenitalia website says this regarding the duration and validity of a train ticket:

La convalida e la durata
Di regola, la validità oraria del biglietto decorre dal momento della convalida mediante le macchinette obliteratrici di stazione. Il viaggio deve concludersi entro 24 ore dall’ora della convalida (in Trentino entro le ore 24.00 del giorno di emissione). Per percorrenze fino a 200 Km il viaggio deve essere terminato entro 6 ore dall’ora della convalida (in Lombardia entro 3 ore, per percorrenze fino a 50 Km).
I biglietti regionali acquistati e stampati via internet sono già convalidati con la data e l'ora di partenza richieste dal cliente e hanno una validità di tre ore. Il viaggiatore che si trovi in treno allo scadere del termine di validità del biglietto può proseguire il viaggio già iniziato fino alla stazione di destinazione, senza effettuare fermate intermedie.

The same website translates the above as:
Validation and duration
The time-length validity of your ticket starts from validation by stamping them in the dedicated machines ("obliteratrici") at the stations.

The journey must be completed within 24 hours from validation (in Trentino, by midnight of the day of issue). For distances longer than 200 km, the trip must be completed within 6 hours from validation (in Lombardy within 3 hours, for distances of up to 50 km).

Does "Per percorrenze fino a 200 km" translate as "distances longer than 200 km"? It seems nonsensical that you have 24 hours to complete your journey unless it's more than 200 km in which case you have 6. How does this work?

quote:

Does "Per percorrenze fino a 200 km" translate as "distances longer than 200 km"?

No "fino a" translates as "up to"

Your ticket allows you, it maintains its validity, for six hours from the time you have validated it .

For example if you start your travel in Turin going to Milan [roughly 140 km]
you have 6 hours to reach your destination.

That means you could stop in an intermediate station,let's say Novara, and then board another train to complete your trip, provided you will be in Milan within 6 hours from the validating stamped date on your ticket.

[in Lombardy further limitation: 50 km within 3 hours with both start and end of the trip within the region borders ]

That's true unless you have bought a ticket for a train for which a reservation is mandatory.

I think the train pages Alice wrote for the main site cover your question.

Thank you so much!

And here's a link to Alice's comprehensive train pages.

Cheers,