There are some very long surahs in the Qur'an, such as Al-Baqarah, Al-Imran, and An-Nisa. While I have read some hadith, such as this one, about such long surahs being recited during obligatory prayers during the period of revelation, presumably it is not practical for most people to do so at present times due to busy work schedules.
If a surah is being recited during a rakat of one of the five obligatory prayers, is it necessary to recite it in its entirety or it is acceptable to recite only a part of it, such as the ayat ul-kursi? If it is acceptable to recite only a portion, what are the rules governing what length must be recited in order for the prayer to be valid?