What time to arrive at train stations?

Twenty minutes should be enough time for you to find your train number on the electronic board (sometimes not assigned a track until 20 minutes before departure), go to the track (binario), locate where your carriage will be (overhead boards will light up), and board the train.

I assume you mean Italo, not Trenitalia.

I like to arrive to the train station at least 30 minutes prior to departure, especially if I'm not familiar with that particular station. You'll need time to get oriented, to check the electronic board, & to get to the correct platform.
Your train information may be posted on the electronic board 15- 20 minutes prior to departure.

Sometimes I arrive much earlier if I want to grab a sandwich or a snack at the station for a long trek.

Enjoy!

There are a few trains that leave from a group of platforms that's waaaaaaay past the main section (e.g. the train to Assisi). It's at least a five minute walk from the main platforms to those platforms. They're called something like 1 EST and 2 EST.

Here's a link to where those platforms are, in the lower right of the Google Maps image, with a pin called "Via Marsala".
https://goo.gl/maps/pDce7746MoC2

I would say at least a half hour. We bought our tickets for Orvieto through the machine. It was quick and easy. It had the time, but not the platform number, as they waited until only 10 minutes before to post it on the electronic board. It was a little stressful for us, but the frequent, more local runs do this.

A couple of other points - Know your train number (that is what is on the board) and the end destination. The board will often list SOME of the in between stops but don't panic if your stop is not listed as long as you have the correct train number and it will be on your ticket.

Second, we often wait at the station end of the platform and don't go down the platform until we can see for sure that our train is at the platform. It happens infrequently BUT the platform can be changed at the last minute. If an announcement is made and the people around you starting moving away, check the departure board quickly. Probably been a platform change. And don't be afraid to ask someone on the platform is this is the correct location. Even if they do not speak English, show them you ticket with a question look and point to the edge of the platform. Saved us a couple of times.

Most trains won't arrive much earlier than 20 minutes or so. Check the departure boards for your train number.

Donna

If it's a big train station, I like to be 30-60 minutes early, but I am a train dork and love the train stations as sites and cultural experiences in their own right. If it's a countryside one, 15 minutes. Roma Termini, probably 45 minutes, just to take it all in, get a snack and so on.

The Freccia and Italo trains are all high-speed trains, and high-speed trains generally depart from the center tracks at Termini. The chance of it departing from an outlying track is very small (if not zero). The track number sometimes isn't posted until shortly before the train arrives, so I suggest you go through the security gate and wait in the area near the center tracks. You can then get to your train quickly once they post the track number. If your train is anything other than a high-speed train, add a few mintues to your arrival time on the chance your train leaves from an outlying track (at Termini, that also includes those numbered higher than 24).

I like to get there about 45 min before. I don't regularly take trains, or public transport and I don't like to feel rushed or like if one thing happens I will be stressed. We also like to grab a sandwich, water and treats for longer journeys. I also like to feel a little orientated. My first time at Termini I was really overwhelmed, and that does not happen to me that often and the bathrooms were down an escalator etc...this last time I felt more comfortable with the whole journey.

and wait in the area near the center tracks.

Wouldn't waiting near the intended track for that train be easier?

There are only 2 possible layouts of European stations: head/terminal and pass through. The tracks inside head stations are always at the same place. What changes is the distance between the main doors and the trains. Maybe you'll have to walk through a shopping center or take a couple of escalators, but if you keep on going straight with your back to the main doors, sooner or later you'll see the tracks in front of you.

Since you have a reservation for your train, I would say being inside the station 30 minutes before departure is enough. If you have any confusion, show your written reservation to a uniformed train official or information person, and they should be able to point you in the right direction. Be sure you understand your car assignment correctly, sometimes there are extraneous numbers on cars that can be confusing.
Now, if you are arriving for a train WITHOUT a ticket, I think it is a different story. Often there are long lines at staffed ticket counters, even at smaller stations, and it may be many minutes before your turn. Sometimes I have also had problems at ticket machines that won't take my credit card, or can't make change. I did once miss a train in Italy for this reason. For this reason, I always try to buy even regional train tickets the day before, or early morning on the day of travel.
Also, should you happen to have baggage stored at the station, especially a large station, give yourself plenty of time to claim it, as lines can be long there too.