We done this trip from Trieste last year. You will get on to Venice Island about 11am, then have approx 4 hours before you get the boat back to the mainland.
Gondolas were a fixed price 80€ with no need to pre-book.
There were also "large" queues to get access to specific buildings.
The 4 hrs allows you to take in the sites, ie up to Rialto bridge, trip on Gondola, have lunch, browse the shops etc. I wouldn't fancy trying to stand in a queue for the likes of Doge's Palace, then have to run around to fit everything else in. If you skip lunch you may manage it, but its a long day once you leave the boat.