Principal photography is that phase of producing an episode in which the majority is filmed. It is the part of producing an episode in which actors are most heavily involved. It generally does not include any special effects, but usually does include practical effects. It also does not include things like pick-ups or re-shoots of completed scenes, both of which are ordered by the production team after principal photography has already "wrapped".
The amount of time taken to complete principal photography on an episode of Doctor Who has varied greatly over time, but the length of time required to complete each story has grown over the years.
1963 - 1989
In the Verity Lambert era, most episodes were completely taped in a few hours, largely because they were photographed sequentially as theatrical plays with minimal editing. As the editing of videotape became more affordable, Doctor Who was filmed out of sequence, and thus more time could be spent filming each scene. Location filming also became increasingly common. From the advent of significant location filming in the waning half of William Hartnell's involvement with the program, principal photography was split between "studio days' and "location days". The total time needed to complete most episodes naturally increased for that reason alone.
There were three major patterns in the program's studio recording schedule. From An Unearthly Child to Spearhead from Space, the studio recording for stories would generally happen once a week. Material for only one episode would be completed during each day's recording. Until The Tenth Planet, this day was invariably Friday. From William Hartnell's last story until Jon Pertwee's first, studio recording was done each Thursday.
While the reduction of the show's punishing year-round cycles of shooting to a more concentrated "season" form in 1970 is often noted, other shifts in practice implemented by the new producer were also underway. Barry Letts introduced a new principal photography schedule with Doctor Who and the Silurians. He decreed that the studio work for two episodes be completed on consecutive days each week. Thus, instead of it taking a month to complete the studio work for an entire four-part episode of Doctor Who, it would now only take two weeks. The innovation was born of necessity. The outside contractor Barry Newbery had used to build the cave sets for Silurians had delivered flimsy sets. Because they would be damaged by striking them each week, Letts came up with the solution of recording two studio days back-to-back, greatly reducing the number of times the sets would have to be taken down and put back up. The plan kept the cave sets from disintegrating. The patten of once-a-week studio filming never returned to Doctor Who.
The Masque of Mandragora — or, to put it another way, Season 14 — brought with it a significant, lasting change to the progress of principal photography. The idea of always recording on Fridays and Saturdays was abandoned in favour of a sliding schedule of two studio days one week being followed by three the next. This not only added a fifth day of studio recording, but also allowed for a whole month to be used. Location filming would usually be done for three days some time prior to the studio recording. In the case of Mandragora, this location filming occurred two weeks before the start of studio recording, which meant that the total time elapsed for principal photography was five weeks. This schedule was retained for the rest of the original series, although stories with fewer episodes, such as Survival, were completed in less time.
The TV movie starring Paul McGann was the first "modern" production of Doctor Who. Filmed as a movie, it had a long and complicated shoot which lasted nine weeks. As a one-off production, it did not establish any particular patterns or continue any legacies from the past. It undoubtedly established the record, however, for the longest principal photography of any single Doctor Who story. However, depending on one's views of what constitutes a "story", it would be bested by later, multi-episode arcs of the next incarnation of the program.
2005 - present
An episode of Doctor Who during the Russell T Davies era of the program typically had a long principal photography schedule with greater commonality to the TV movie than the original series. However, it is difficult for those not on the production to speak intelligently about what the actual schedule was, since individual episodes were almost invariably described to the press as "behind schedule". Nevertheless, Rose was likely fairly close to meeting its schedule; it took about two months of almost around-the-clock principal photography. Principal photography was achieved in the RTD era through the use of recording in production blocks, comprised of anything from one to four episodes, with photography from one episode sometimes overlapping with that of another. In the case of the Cyberman arc from series 2, for instance, action from all four episodes was filmed jointly over the course of about three months, although material from Rise of the Cybermen/The Age of Steel generally took up the first six weeks, while Army of Ghosts/Doomsday was mostly recorded in the latter six weeks. At the other end of the scale, an episode like Fear Her, with limited sets and location work, had only about three weeks of principal photography.
Efforts to alleviate the risk of missing key production windows and the strain on cast and crew led to the adoption of double banking strategies. The show would ultimately relinquish its 13 + special(s) cycles, reducing consecutive runs of the show first to 12 (2014) and later 10 (2018) episode stints.
Despite this, the key difference between the original series and the modern one remains evident. Whereas "classic" Doctor Who was filmed at a slow, weekly pace, the Davies, and later Steven Moffat and Chris Chibnall, eras are characterised by long, gruelling periods in which work is completed every day for weeks on end.