I am starting to plan how many weeks I should spend in the countries I have chosen for a trip...
If I want to feel like I "have seen" the Czech Republic, how many weeks do I need there??

As with Slovenia, I'd say two weeks.
A lot of course depends what you're interested in doing - hiking, museums, sightseeing, culture etc., but I guess two weeks would be fine.
Edited by: mattoni
#1 is right that you should be able to see all the highlights of the Czech Rep in two weeks. On the other hand of course, you could spend months exploring the country. After all it's got A LOT of beautiful towns and you might want to enjoy its scenic national parks too.
Personally, If time was not short, I'd go one up and say three weeks for (almost) the full picture.

Like I wrote in the post before.. but I can tell you one think, I lived in czech republic two years and still cant say that I have seen it all..

I've been living in the Czech Republic for four years.
Give yourself a minimum of two weeks to properly see the high points here. One week in Bohemia and one in Moravia.
If you can give it two and a half or three weeks that's even better to get into the smaller places that are still interesting and explore the Vysocina highlands between Bohemia and Moravia a bit..

It depends on your style of travel. You can see much more if you travel by car. Czech Republic has many great sites(castles, chateaus, small towns) that are time consuming to reach by train or bus. To give you an example I've seen the following places in 1 day by car: Holasovice, Ceské Budejovice, Hluboká nad Vltavou, Cervena Lhota, Telc, Vranov nad Dyji. To see all these without a car takes at least 3 days!

A further note to what pszeker has said:
In my experience Cervena Lhota is all but impossible to access easily without a car (some go in by bicycle). Bus service to it is irregular and train service to it non existant.
An additional thing to bear in mind, if you plan to visit the caves of the Moravian Karst, the Punkveni Cave requires a reservation. Punkveni and the associated Macocha Abyss are the most popular part of the karst and if you're visiting in high season, you may need to reserve a few months in advance.