You will need approximately 25 minutes (16 minutes by express train CAT) to reach the city centre of Vienna. I would rather take the MTA train S7, because it's cheaper by far (EUR 3,00 for a single trip) instead of EUR 9,00 for the CAT.

Schedule of MTA train S7 from the airport:

09 and 39 minutes past the hour

Schedule of CAT from the airport:

05 and 35 minutes past the hour

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Schedule of MTA train S7 from station Wien-Mitte:

26 and 56 minutes past the hour

Schedule of CAT from station Wien-Mitte:

08 and 38 minutes past the hour

If you decide to visit Vienna, I recommend to proceed towards the 1st district
by taking subway line U3 to Stephansplatz, where the St. Stephan's cathedral is located. You can walk around on Kärntner Strasse, Graben, Kohlmarkt to Michaeler Platz, where the Hofburg Palace begins. There you find the Cafe Griensteidl, a typical Vienna coffee house.
After you have enjoyed your freshly brewed Wiener Melange, you can take the subway from Herrengasse back to Wien Mitte.
This little walk trough Vienna should be possible, but five hours are too much to remain at the airport, and too less to see the sights.
It's up to you, just make your decision upon arrival.

If you have any further questions, don't hesitate to contact me.