Hi @Stacey L McCrath-Smith25 welcome to the community!
The Leonardo Express tickets are valid for 4 hours from the time you choose.
I have actually got through several times from my plane to the platform in as little as 20 mins (yes true!) but usually 30-35 mins. with no checked bags.
I would book a train that is 30 mins from your arrival time and it will be valid for 4 hrs after that in case you need more time :-)