In classic Doctor Who (1963-1989) most stories consisted of four 25-minute episodes. There were a couple two-parters and a couple six-parters, but usually it was four. In the US, though, they were often shown all at once like a movie. One local channel aired them in their original 25-minute format weeknights at 7:30, but I hated watching it like that. I preferred getting the whole story all at once.
The current series episodes are 45 minutes, but they often tie together, so that two or three episodes make a complete story arc. I prefer the current system.