Devil fruits will spawn every 30 minutes under or on a tree in a random location in the map. When a devil fruit first spawns, the fruit will stay for 30 minutes before de-spawning. On top of this, even if you pick up a fruit before the de-spawn time it will leave your inventory after the 10 minutes are up. This basically means you have to find the fruit and eat it as soon as you pick it up otherwise it'll disappear. Assuming your not near the target island, the fastest way to get to the fruit would be using the Pika-Pika No Mi's incredible flight speed or with the Coffin Boat. There is only one way to know when and where a fruit has spawned, and that's with Fruit Position. Currently there is no known pattern to where a fruit will spawn next, and not only that but what the fruit will actually be is also impossible to predict. Due to all these reasons, finding a devil fruit is not only very difficult but also impractical making this the worst way to obtain devil fruits.
The easiest way to locate devil fruits when they spawn is with the game pass Fruit Position. With this, it'll notify you when a devil fruit spawns. A message will pop up on your screen with a sound effect that says a devil fruit has spawned, and shortly after a trail of blue arrows will lead you to its direction. Fruit position will still be active constantly and can't be turned on or off. An important thing to keep in mind is that Fruit position will not tell you where the arrows will lead to, the arrows are pin pointed on the Devil Fruit, not the tree where it spawned or the island. This means that while the arrows will lead to the fruit, you yourself will have to figure out which island it's actually pointing to.
Soldier Island/Shells Town
Shark Park/Arlong Park
Snow Island/Drum Kingdom
Lobby Island/Enies Lobby