My wife & I are planning a trip to the above cities May, 13. I was thinking 7 days each but is that too much time? Maybe too much for Prague but okay for the other two? We don't mind taking a break from sightseeing so I'm thinking at least one day each for recharging.
Any other suggestions would be appreciated as we may expand this to a month total

You'll need to set aside most of 2 days to travel between cities so you don't have 7 days in each city. It's a good amount of time and will give you time to sightsee outside the cities. You can take day trips from Vienna to Melk Abbey and perhaps do an overnight to Salzberg.

Between Vienna and Prague you can stop for an overnight in Cesky Krumlov and then from Prague do a day trip to Kutna Hora.

Budapest also offers day trip options.

If you have a month that would be great. You could add one of my favorite cities, Krakow. Or end your trip in Berlin, also a good place for a week with day trips. Stop in Dresden on the way from Prague to Berlin.

There are lots of options.

You might want to add a couple nights in the St. Wolfgang and/or Hallstatt area.

I agree about Cesky Krumlov and Melk!

From Prague, you may also want to add Terezin as a day trip. A very sobering but worthwhile day.

You do NOT have too many days - since there are at least one or two great day trips (by train, bus or boat) outside the city. From Vienna I would definitely take a day trip to the Abbey at Melk - as well as the ones mentioned above.

Frankly I like Prague the best of the the 3 - although Vienna may have slightly more to see/do - but it always seems quite triste to me - while Prague has a much fresher and livelier feel.

Thanks all. Feel better about not "wasting" time by staying too long in one place. One week each sounds great.

Yes, you're right. One week is excellent for each city, and you also have time for day trips.

http://en.wikipedia.org/wiki/Durnstein Durnstein would be a lovely stop on the Danube. More info www.inyourpocket.com/Austria/Vienna and www.inyourpocket.com/Czech-Republic/Prague and http://en.wikipedia.org/wiki/Budapest