Air (straight line) distance from Prague to Český Krumlov is about 140 km (87 miles). Here you can see where Český Krumlov is located on the map.
Travelling on the road it is about 170 km (106 miles) via Tábor (D3 motorway) or 184 km (114 miles) via Písek (D4 motorway). The former is usually a little bit faster overall, but it always depends on traffic.
Travel time from Prague to Český Krumlov is about 3 hours with the fastest bus services. Add more time if changing buses in České Budějovice.