Several thousand years
When Klak was inside of Caiaphus's brain, a supposed future form of himself attempted to stop him. He convinced his "past self" to become evil to save their friends. However, once Klak went back to the light, this 'future' version became part of an alternate timeline, that will only become reality if Klak chooses to be evil once again, according to what some say. F-Klak once looked like a mix of F-Klak's normal form with his Nova beast form, but now looks like Forte from the Megaman Battle Network series. F-Klak was thought to be a being from an alternate timeline, but he turned out to be a clone of Klak.
Apparently, the events of the Uterio War impacted Klak in a certain way. Since his arrival on Korasa was not long after the war's end, his psyche was ripe for the unstable cloning process. Klak was knocked out, then woke up, realizing that he had a clone on a bed nearby. This clone eventually escaped the facility, and killed Commander Yammark in cold blood. The clone was clearly deranged and unstable. This will cause him to begin a conquest of the universe, tricking his comrades into thinking that he is the real Klak, and killing many of them. Eventually, F-Klak will have grown in power, and will have either killed or overpowered the other major characters. A significant part of the Original Universe was conquered, and he would still wage war to conquer more.
F-Klak would spend his days from then onward making sure that his empire was at the size he wanted it to be. He began to mess with the past for his own amusement in his free-time. Examples of interference include his speech to Klak inside Caiaphus's brain, the Battle of Malchior IV, and the Vampire Plague that affected that very same world.
Several millennia later, (in an epic known as Fall of a Hero: Ynot's Farewell) F-Klak confronted the newly arrived Ynot, but his fleet was destroyed. The dark Makuta survived, but woke up to find that the Users who had practically protected his empire were gone, and Blackout gained their power. This caused enormous chunks of his empire to be lost. The rest of it was exiled to what is known as the "Dark Zones". F-Klak began to rebuild his empire, and even took over something similar to the Green Lantern Corps.
By the time of the epic, BZPB: The Lesser of Two Fates, F-Klak has prepared his troops for an assault to retake from Blackout what was once his, the Original Universe. F-Blackout was attempting to prevent F-Klak's manipulation of time while attacking some of his villages.
F-Klak traveled back in time with Striborg for a final conflict with Klak, thus removing a potential thorn in his side. However, in this conflict, he found the original Klak to be more of a match, especially now that Caiaphus was fighting them both. The massive conflict, known as Ragnarok, resulted in F-Klak's death. In the far flung future, his empire collapsed into infighting between his soldiers, the Lantern Corps, and the Resistance movement. It is unknown what will occur in The Reckoning.
Abilities & Powers
F-Klak boasted immense power, being able to strike fear in several powerful characters, even Namine. She only was able to defeat her former master in combat because he wanted her too. According to User:Klak-a-Klak, he was powerful enough to take on both Blackout and Zev at the same time, and win. By this time, we can assume that F-Klak (when he was created, he was known as Klakk) has been able to train and study the shadows to become the most powerful Makuta in the multiverse.
So far, his powers included all 42 Kraata powers, Key Blade mastery, telepathy, and time travel (and time traveling telepathy).
Is he Klak's future?
The ZFT book seemed to have supported this, but David Robert Jones lied. F-Klak is not the original Klak, but a Cadmus clone meant to be his replacement. However, the clone went mad as it had all of Klak's postwar mental instabilities, and went after the shadows, turning into a tyrannical monster. It is unknown what Books LLC will do about this.