During the events of The Hobbit, as Bilbo arrived at the Lonely Mountain, Smaug the Dragon had been sitting in there for a staggering 171 years. During this time, he protected the stolen treasure horde closely, and no living being dared stray too close. However, this has raised questions about what Smaug ate and drank, as the foods of Erebor likely wouldn't have lasted for over 100 years.
Smaug had no problem eating Dwarf, and on the day he attacked the great Dwarven kingdom, he likely had enough bodies to last him a long while. But over the decades, the bodies would've decomposed, leaving only whatever meals the Dwarves left behind. And for a beast of his size, it's safe to assume he hungered for large quantities of food.
Yet, with Smaug being a dragon, he may have shared the same traits as real-world reptiles -- most notably, their ability to go long periods without eating. Many reptiles have a form of hibernation called brumation, where they'll slow down to conserve energy over winter and not require any food. In extreme cases, species like the crocodile can go up to three years without eating, remaining mostly still during that time.
When Bilbo makes it to Lake-town, it's noted that no one's heard from the Dragon in years. And when Smaug is found, he's sleeping motionless in his mountain of gold, meaning it's very likely that he can brumate for years at a time. However, there's a common misconception surrounding Smaug, and it's the assumption that he never left the mountain.
With a faster passage of time, The Hobbit movies make it appear as though Smaug has hidden away ever since he claimed the treasure. But the first chapter of the book proves this wrong, as Thorin explains that the front gate is too dangerous and that "out of it comes the dragon... far too often, unless he has changed." And it's also noted that the neighboring city of Dale wasn't immediately evacuated, as "Later he used to crawl out of the great gate and come by night to Dale, and carry away people, especially the maidens, to eat."
And so, it's confirmed that Smaug used to pick people from Dale to eat, although it's not known for how long. It could be that this was enough to sate the dragon for over a hundred years, but it's more likely that he moved on to the next-closest population: Lake-town. As one description of the town reads, "It seemed a town of Men still throve there, built on bridges far into the water as a protection against enemies of all sorts, and especially against the dragon of the mountain."
While it's not explicitly said in The Hobbit that Smaug took people from Lake-town, the fact that they built defenses suggests it was likely so. Thus, Smaug probably had more than his fill during his stay at the mountain. And while he might've been able to go years without food, there were still nearby woods and fields to pick fresh game from, as he would probably eat any creature available to him.