Well, that is a long story and depends on whether you mean "Official" naming or common name use. The common use of Czech for the region is mostly a 20th century thing. The creation of the country "Czechoslovakia" dates to the end of WW I, and I suppose this is when the region started to get thought of as "czech", although Hitler wasn't buying it. After WW II, the country was reconstituted in a slightly different form and here we are, now.
The peoples of the region are a mix of different folks, and Bohemia is really simply an older Germanic group name for the people in part of the region, but the population has been somewhat fluid so there isn't a definite "bohemian" or "czech" people. They have slightly different historical origins but have occupied much of the same region. The days of the little kingdoms like the Kingdom of Bohemia are long in the past, and even that kingdom wasn't a constant or truly specific thing through time.