Terezín Day-Trip is a real time commitment — fit it in only if it's a priority.
Allow Half to full day (5–7 hrs).
A profound, important half-to-full day — the ghetto museum, the Small Fortress and the columbarium make the Holocaust devastatingly concrete, and a guide deepens it immeasurably. The honest catch: it is emotionally heavy and not 'enjoyable' in any normal sense, logistics (museum + fortress are apart) need planning, and it's wholly unsuitable for young kids. Go for understanding, not sightseeing.