Buffer time was a means of creatively estimating or exaggerating how long it takes to complete an assignment. A lower decks tradition, it was built on the premise that command level officers had no idea how long it took to complete a task, combining the "you never admit the actual amount of time it takes to finish a job…" so that "you're a hero when it's done early." This allowed the crew time to relax between jobs. (LD: "Temporal Edict")
Montgomery Scott was infamously known for his exaggeration of repair times, often multiplying the estimated time it would take to effect repairs by a factor of four. He admitted as much in 2285 after Rear Admiral James T. Kirk, to whom he'd just promised to be able to complete a supposed eight week task of repairing the USS Enterprise in two weeks, suggested he may have always done so, adding that he couldn't keep up a reputation for being a miracle worker if he didn't. (Star Trek III: The Search for Spock)
Scott later advised USS Enterprise-D chief engineer Geordi La Forge in 2369 that he had a lot to learn if he wanted people to consider him one, too. This comment was made after La Forge admitted he had been honest to Captain Jean-Luc Picard about how long a repair would take rather than exaggerating it. He rationalized this approach, explaining "Starfleet captains are like children. They want everything right now and they want it their way, but the secret is to give them only what they need, not what they want." (TNG: "Relics")
Like La Forge, Lieutenant B'Elanna Torres was honest about her estimates, as she explained she would be ready the following day, when given the task of setting up an expander on the bridge of a Kazon warship in 2371. When Captain Kathryn Janeway told her she wanted it ready by the end of the day, Torres affirmed that "[w]hen I say tomorrow, I mean tomorrow. I don't exaggerate. Tomorrow is the best I can do." (VOY: "State of Flux")
In 2372, Miles O'Brien gave Worf a few pointers on engineers, specifically Stevens and Muniz, stating that "they're used to being given a problem to solve, then going out and figuring out how to do it," and that the overly strict Worf should "[g]ive them a little slack. Ease up on the reins. Let them do what they're good at, and give them a little encouragement now and then."
Worf later took O'Brien's advice under advisement, and told them to proceed on their repairs of the USS Defiant at their discretion. Worf was then warned by O'Brien not to "take your hands off the reins," after the two engineers told him that the repairs would take sixteen hours, when O'Brien, who was present during the report, knew full well they could do it in twelve. (DS9: "Starship Down")
The concept of buffer time was accidentally revealed by one ensign to Captain Carol Freeman of the USS Cerritos, who further pursued information on the subject from Brad Boimler after she heard his reference to the term. Soon thereafter, she instituted the "Temporal Edict", which established time deadlines for all tasks. The ship descended into chaos as the crew attempted to complete their work on time, and when an away team arrived on Gelrak V, they accidentally brought the wrong gift due to the panic and disarray.
In anger, the people of Gelrak V attacked the Cerritos. The crew was unable to respond until Boimler reminded Freeman that they needed to be able to complete tasks in their own time in order to do them well. Freeman, realizing her mistake, brought back buffer time and got rid of time deadlines and the crew was able to focus on repelling the attackers. Freeman later instituted a new mandate named "Boimler Effect" that encouraged crewmembers to take shortcuts and not blindly follow the rules and to build in buffer time whenever they deemed fit. (LD: "Temporal Edict")