Venice, St Marks Square to Frari Church via Rialto Bridge

Is there a reason you want to crisscross Venice this way?
The easiest way would be to take the vaporetto. From at marks head to S. Marco (vallareo) sp. ferry dock to the s. Toma stop. Walk around 3-4 blocks and you're at the Frari Basillica. From there walk to the Rialto Bridge. What you'll be missing going this way is hordes of humanity on the other side of the Grand Canal.

It depends on crowds, how fast you walk, and whether you can avoid getting lost in San Polo! We have spent many weeks in Venice, and I can easily navigate my way around all the sestrierei---except San Polo. There is something about the way the callers and canals are laid out---it is hard to find a through route. But maybe you will have better luck. Google navigation might work, but I refuse to use it in Venice.

Since time is limited, you might consider the vaporetto from San Marco to the San Toma stop, which is pretty close. Or better yet, assuming it is still running, would be the traghetto that crosses the Grand Canal near the San Toma stop. We used it in 2013 but I have not checked on it lately.

Google maps works great in Venice. Used it several times while there in June. Even the vaporetto schedule is accurate.

If you don't get lost it would probably take 30-60 minutes.