Milan metro ticket

24 hours after it's first stamped.
Yes, there is a 2-day pass.

https://www.atm.it/en/ViaggiaConNoi/TurismoMilano/Pages/Biglietti.aspx

More info:
https://www.italylogue.com/planning-a-trip/how-to-get-around-in-milan-milan-public-transportation-101.html

Yes, there is a 1-day ticket for 4.50 EUR and a 2-day ticket for 8.25 EUR. These day passes are valid for 24 or 48 hours after first stamping. You can buy these at most Tabacchi, which are news/cigarette shops all over town, as well as the metro ticket windows and vending machines. There are lines at ticket windows at major train stations and "helpers" stake out the vending machines asking(demanding) for tips to help you navigate the machines. That is why I suggest using a Tabacchi.
https://www.tripsavvy.com/what-is-a-tabacchi-in-italy-1547549

Milan's Transit Authority is called ATM. They run the metro, the buses and the trams (includng the trolley cars from the 20s that the Milanese love and that are as protected as Panda bears in China).

Therefore, all ATM passes can be used on all metro lines, buses and trams.

Thanks to an agreement with railway companies you can use the same passes also on local trains running within the city limits. It's the underground "Passante" railway tunnel, the dark blue line(s) on metro maps.

Please, to avoid fines note the difference between urban and extra-urban tickets explained here: www.atm.it/en/ViaggiaConNoi/Biglietti/Pages/Tipologie.aspx

I think you can load tickets and passes also on a smarthphone that can display a QR code. You can also use a contactless credit card to pay for a single metro ride and open the turstiles.

Thank you everyone. I bought the 48 hour pass. Much appreciated of all the feedback and suggestions for this entire trip -visited 3 countries.

We are in Milan now and appealing it’s the fashion week, busy but worth visiting and doing last minute shopping.

Cheers