How Long Is the Train Ride: Milan, Venice, Pisa, Rome?

You can look these up on one of the easier sites to use
https://www.trainline.eu/

Look at the official train website: http://www.trenitalia.com

If you need exact times, use the suggested websites in the other responses. If you just need an approximation to make a decision where to go, then they are all approximately 3 hrs, some routes will include regional trains, some will he high-speed, number of train changes will determine the exact times on each route.

Thanks everyone! At this time, I don't need an exact time. i just need an estimate as to how long it takes to get from one point to the next.

If you use the desktop version of this site, our "host" Rick is famous for the amount of free train-travel advice he gives out. You will find maps for each country with estimated train times, under his Travel Tips menus. (Top left, blue logo).

Samirah, if you are still in the "learning phase", I suggest you read this webpage: https://www.seat61.com/Italy-trains.htm
It is the best primer on travelling by train in Italy on the web.

P.S. most people would do a round trip, for example: Fly to Milan --> Venice --> Pisa (What happened to Florence?) --> Rome, fly home from Rome.
Get a map of Italy (click here) and join up the dots in whatever order you want. Note this map shows Italian names for places (Milano, Venezia, Roma), which is what you need to use when you start looking up trains.

I suggest the Rome To rio app. It’s very helpful for planning

The bahn.de site has schedules; when you are ready to purchase tickets, use Trenitalia, Italotreno, or trainline.

Rome2rio.com is very good for planning routes, distances, has links to transportation sites.

I use Rome2Rio a lot as a first step in deciding whether I'll need to used a bus rather than a train, or perhaps a combination of the two; it also covers flights. I've found it not consistently reliable about duration, frequency and cost of public transportation. I wonder whether that is caused partially by timing issues--does it matter what time of day you look something up? In any case, I wouldn't rely solely on Rome2Rio. I normally drill down to the link it usually provides to the appropriate train or bus company, use the Deutsche Bahn for train schedules, or Google "Bus Padua to Bolzano", etc. The latter is usually, though not always, effective for finding bus schedules.

Bus schedules can vary significantly from month to month, so it's essential to be sure you're looking at the appropriate schedule.