In the classic series (1963-1989), most episodes were 25 minutes, with stories normally consisting of 4 or 6 episodes (occasionally fewer & occasionally 8 or even more). Some special episodes were longer: "The Five Doctors" (20th Anniversary Special) was 90 minutes.
"Doctor Who" (the TV movie), in 1996, was 85 minutes.
In the revived series (2005-present), most episodes are 45 minutes, with stories normally consisting of 1 or 2 episodes (rarely more). However, some episodes have been 50 minutes & Specials have been 60 minutes or longer. There have also been several "minisodes" lasting only a few minutes.