His feasting time lasts about 10 months. Then, he goes back to hibernating for 27 years until it is his time to feast again. The cycle continues until the Loser's Club defeated him for good the second time they fought him.
The character is an ancient, trans-dimensional evil entity who preys upon the children (and sometimes adults) of Derry, Maine, roughly every 27 years, using a variety of powers that include the ability to shapeshift, manipulate reality, and go unnoticed by adults.
A powerful and centuries old being in the form of a clown, Pennywise would awaken in many 27 year cycles of terror, feasting on the residents of Derry, Maine until going back into hibernation deep beneath Derry.
This fierce fear demon gorges itself upon that which terrorizes its victims, using said fear as a weapon before killing them. Of course, the entity's most common form is that of Pennywise, the Dancing Clown. Due to this preferred diet of adolescents, It comes back every 27 after feeding to consume a new generation.
Related: What Does Pennywise Really Look Like In IT? IT arrived on Earth through an event similar to an asteroid impact, landing in what would later become Derry, Maine. Once there, IT adopted its usual pattern of hibernation that lasted between 27 and 30 years, awakening to kill and eat and then going back to sleep.
However, It must surrender the the laws of whatever shape It takes. Pennywise's strength is also his weakness. For example, if he were to take the shape of a werewolf (as he does in the novel), silver bullets would harm him.
Also in the 2017 film, Georgie's death is changed. As well as having his right arm bitten off, Georgie is trying to crawl away but he was dragged into the sewers and devoured by Pennywise, with an unnamed older woman and her cat being the sole witnesses of the horrific scene, including before Georgie lost his arm.
According to It, when humans got scared, "all the chemicals of fear flooded the body and salted the meat". This is why he prefers to feast on children -- their fears are simple, pure, and powerful compared to the complex, pathological fears of adults. Basically, children are delicious.
In their final battle, the Losers' Club was able to kill Pennywise by reducing him to a child-like form as they overcame their fears. In the moments before Pennywise died, his fear is reminiscent of Georgie's own, therefore closing the cyclical nature of Pennywise's murders.
And it is not totally true that adults can't see Pennywise because there are several incident in books where adult see Pennywise but adult don't find it as a threat because of the reason that Pennywise choose not to. Pennywise only attack children as they are easily manipulated and easily scared.
Stephen King's IT introduced readers to a one-of-a-kind creature that can take any form, the most common one being Pennywise the Dancing Clown, and as menacing as it is, this creature has one big enemy it's truly scared of: Maturin, the turtle.
Though there are some IT characters just as scary as Pennywise, the film's main villain proves to be its deadliest, with a kill count that actually reaches into the tens of thousands. Reddit user u/angelholme estimates that Pennywise actually took the lives of between 12,117 and 18,011 people.
"And the moment you see her performing, you know this is a serious actor." Not only could Gregson bring the convincing warmth, but she turned on the menace as Bev figures out Mrs. Kersh is the daughter of Bob Gray, whose evil clown Pennywise is an incarnation of the shapeshifting creature referred to as "It."
As Steve says, the creature fell to earth as a kind of a meteor during a prehistoric era, which would make it a few million years old.
In the novel, It's origins are nebulous. He took the form of a clown most frequently, Mr. Bob Gray or Pennywise, but his true form is an ancient eldritch entity from another universe who landed in the town that would become Derry by way of an asteroid and first awoke in 1715.
It was said on page 1361 that the Spider being both female and pregnant was a symbolic interperetation. Its not literal, but a representation of whatever equivalent but inconceivable reproductive capability It actually has. Pennywise didn't become pregnant.
The spider-clown shrinks as the Losers hurl taunts at It, until it's tiny and weak enough that they pluck out its heart and squash it into nothingness. In the end, they defeat Pennywise by, uh, making him feel really bad about himself. It author Stephen King.
If Georgie hadn't died, the Losers Club wouldn't have fought Pennywise and come back 27 years later to finish IT off. Nothing really, if Bill's brother didn't die they wouldn't try to find out what happened to him.
A neighbor witnesses his disappearance, but the fact his body is never found gives Bill hope Georgie somehow survived. Pennywise prays on this in the film, using that hope to lure Bill in the finale. Bill later finds the tatters of Georgie's coat and finally accepts his brother is gone.
Or the creature at least appears to be, as IT's been laying eggs underground. Upon that revelation, the Losers' secondary mission becomes destroying all IT's eggs so that no baby monsters can hatch, in addition to killing their nemesis itself. Thankfully, the book makes it fairly clear that they accomplish this task.
The Pennywise the clown origin story sees an ancient, evil being that is perhaps as old as the universe itself. In the novel, however, the entity IT/Pennywise has a real name — Bob Gray. Bob Gray isn't human, however, and shares the same origin as IT.
The novel explains that IT landed on Earth during an asteroid impact and established itself under the land Derry would be later built on, and initially preyed on indigenous tribes. From that point of view, IT/Pennywise would be billions of years old, but his clown shape wouldn't be that old.