FROM southern part of Sausalito, or 4 th street, and Bridgeway
1) Bus= 45-60 minutes, as one needs to walk from Mission Street to Union Square.
2) Ferry(Sausalito)= from 60-75 minuteds depending on the ferry( will it stop in Tiburon after picking up in Sausalito?) and how you will get from the Ferry Building to Union Square.
3) Bicycle= 35-60 minutes
4) Private auto= from 15-30 minutes depending on bridge traffic, and are we talking about driving in on a friday night or a holliday weekend night?
FROM northern part of Sausalito proper, or near Mollie Stones Grocery store
1) bus=same as above
2)Ferry= same as above
3) Bicycle= add 10-15 minutes
3) Private auto= add 5- 10 minutes.
Consider public transit, expense and hassle can be greatly reduced. My wife bus commutes from Mill Valley to Montgomery/Sutter in ~45 minutes. http://maps.google.com returns travel times at your selected day/time of sorta accuracy. Traffic is variable and complicated by real-time lane allocation on the Golden Gate so 2-45 minutes is probably as close as one can get.
Don't forget the ferries.
Mapquest and Google Maps can give you turn by turn directions from one address to another. They break down the distance at each turn and provide estimated travel times. Typically the times don't factor in much traffic, so rush hour will be another thing, but some days are always better/worse than others. You should get a fairly reliable estimate from there.
Basically you won't have any traffic into the city and over the bridge off commute hours and you may have a bit of bridge back-up during commute times, but minimal. (although because of Doyle drive construction it is worse now than it's been in the past).
Your main commute time will be once you are in SF, but Union Square is "easy" as compared to other places in SF that are across the city.
I would estimate at the shortest (no traffic 15 minutes), at the longest , worse case scenerio 45 minutes (which would have to be really bad). I'd say your average time would be about 20-25.
Hope that helps!
Liz McCarthy, Marin Real Estate Broker