You mean the station building/access to the tracks itself or the counters? Counters can vary but easily googleable if you type. “Prague Hlavni Nadrazi opening hours ticket ofice”.

Most stations (and track access) close at night, a bit after the arrival af the last train and open a bit before the first train of the day.

Other smaller stations, are often not closed off. But they’re often not more than a corridor with street access and stairs to the platforms.

I meant the access to tracks, so thanks for the answer!