The frontier in time was the edge of the Great Houses' noosphere in the universe's far future, past which they could not see or safely travel. The frontier appeared as the posterior end of recordable history in the Spiral Politic.
The concept of a destination being "too far in the future" was meaningless to the Houses; their travel past the frontier was limited only because so many other time-aware cultures were active in the later universe. Since they preferred to believe themselves to be masters of the whole continuum, members of the Houses rarely ventured into the frontier. (PROSE: The Book of the War) The Tenth Doctor said that Time Lords never went as far as the year 100,000,000,000,000, although the Master hid there during the Last Great Time War. (TV: Utopia)
The Doctor's TARDIS once exceeded its time parameters and came to Frontios in the era following Earth's destruction, a time and place at the edge of the Gallifreyan noosphere. (PROSE: Frontios) The Fifth Doctor told Tegan Jovanka and Vislor Turlough that there would be "serious trouble" if the Time Lords knew about his visit to Frontios. (TV: Frontios) At another time, the TARDIS would only take the Eighth Doctor as far forward as Endpoint, and it needed to recalibrate before it could dematerialise again. (PROSE: Hope)
On the map of the Spiral Politic, the frontier worlds were those least linked to the Homeworld, and were constantly shifting into new positions as their roles in history changed. These included Simia KK98 and Ascension. Zo la Domini was a marker of the frontier in time; after a Violent Unknown Event occurred there, Thessalia disappeared at another location on the frontier.
Faction Paradox had no difficulty crossing the frontier and voyaging into the depths of the posthuman period, where they had links with underworld and even terrorist cabals; though the Houses accused them of conspiring with posthumans who scavenged wrecked Homeworld technology, the Faction instead voyaged past the frontier just for the satisfaction of doing something the Houses had made taboo.
During the War in Heaven, the enemy patrolled the frontier and the areas beyond, suggesting that enemy forces may have been in full operation beyond the frontier. Homeworld timeships were frequently repelled from this region of the Spiral Politic, and as many as five were captured or destroyed along with their occupants. Compassion found the crumpled remains of the 103-form timeship Percival on the frontier in time.
When an expeditionary cohort of the Seventh Wave was cut off from Homeworld contact close to the frontier with only an outmoded 89-form timeship, Quintessence Officer Hierarchio predicted their chances of return to be minimal and suggested that they settle on one of the still-accessible worlds. There, the troops visited the Anvil Stars region and observed the birth of new universes and the creation of Leviathans. (PROSE: The Book of the War)