Bratislava from Vienna?

It depends somewhat on what you want to see in Bratislava. I left Vienna about 9:00am and was back there about 3:30-4:00pm. I didn't do a lot in Bratislava, just walked around the old town area (took a lot of photos), did some shopping at the souvenir market, had a wonderful lunch at a sidewalk cafe, then back to the train station and home to Vienna. I know there are lots of things you can see in Bratislava (the castle, a museum, etc) but I just wanted a laid-back relaxing day trip and I was very happy with the experience.

You might find this article from the NY Times helpful.

I would say that mid-morning to dinner time is more than sufficient. Definitely take the bus (not the train) as it will drop you off in the center. The train station is a bit far out and it can be confusing to find your way. The Vienna bus station you should use for this trip is located at Erdberg, which is a stop on the U3 subway line.

There are earlier buses on the schedule but you wouldn't find me on one before 9:15 or possibly even 10:30. Either one gets you to Bratislava before noon, with a good portion of the day to explore.

Depending on the month you're going, you can go to and from Bratislava by a fast hydrofoil ferry. Most people take the bus for the 39 mile trip much of the year.

Bratislava is basically a full day's trip. It is a lovely small town right on the river. We were very surprised to find a very large, modern mall with a bunch of restaurants and sidewalk cafes.

All of Vienna shops in Bratislava - things are cheaper there and shops are open on Sundays.

I've been twice from Wien and liked it a lot - interesting and pretty and unassuming. Why not allocate a full day to it? Why not stay overnight and just see what happens? More time means more focus and attention and opportunity for unexpected discovery.