Which train and how soon can you book tickets for round trip rome to Venice?
120 days in advance. Pick the time you want.
As Sam says, you can book 120 days out for the high speed frecciargento services. All bookings are for a reserved seat on a specific service on a specific date at a specific time. The cheapest tickets are excellent value and sell quickly, but are non refundable.
The best site to use is www.trenitalia.com Go to www.seat61.com for an excellent overview of italian train travel.
It would help to have some idea when your trip will be taking place? Tickets after June 14th may not be bookable on Trenitalia yet, as the new schedules will be uploaded then. However, Italo may have tickets available but you may have to use a different station in Rome.
Trenitalia's mid-June to December schedules will probably load well before June 14, I am checking for tickets almost every day now. The cheapest fares will go fast.