Hi Jim. Here's the page12 from the Marco Polo Venice airport website that details the public transport options for how to get to the train station. Here is some information from that page:
At the Airport: bus stop, second lane outside the Airport Arrivals hall.
ACTV - Route 5 and 156 to Mestre and Venezia
In terms of time, if you're going to the Mestre train station7 (Mestre Stazione Ferroviaria) the ATVO express bus from the airport is a 17-minute ride. If you're going to Santa Lucia station, the express bus goes to the bus depot in Piazzale Roma9 (20-minute bus ride) and the station is a six-minute walk across the canal from there. So I'd round up and leave an hour for catching the bus and arriving at the station.
Hope this helps! Have a wonderful time in Italy.