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While I am not certain on the time Marines and Pirate Hunters can obtain them, I can give you information on the different types. I'll just go from scratch about what the Devil Fruits are, then get into more complicated information:

Rarity: They are rare inside the Grand Line, and rarer still outside of it. Since they are so rare, most people know absolutely nothing about what they are. Oh, and by the way, none of the original 51 fruits (like the Gomu Gomu no Mi, or Gum Gum Fruit) exist in this roleplay, so, once we find a Devil Fruit, we make our own...Provided the power is reasonable, of course.

Appearance and how to obtain the abilities of one: The Devil Fruit, when consumed, gives a power or special trait. They, the Devil Fruits I mean, come in different colors and shapes. All Devil Fruits have swirl marks on them of some kind. There can be only one of each type of fruit. Remember that well, since that means no two fruits are alike, and no two Devil Fruit users will have the same power. Only one bite is needed to gain the power of a Devil Fruit, though swallowing the fruit whole has the exact same effect. Please though, do NOT try to be a smartass with this fact, trying to rob other people of the Devil Fruit without their prior consent, unless you kill them, of course.

Weaknesses: There is one general weakness that applies to all Devil Fruit users: Whoever eats a Devil Fruit will be unable to swim ever again, an effect that is quite crippling to any sea faring person, which, in the case of this roleplay, is everyone. If you fall into the ocean and have no one to retrieve you...Well, usually you're pretty much dead. Devil Fruit users are susceptible to all types of water, not just seawater. "Moving" water, like rain or waves, does not weaken Devil Fruit users, by the way, but standing water does. If you're not completely submerged, you can still use your power from the part of your body that is above the water. Lastly, as Ikilled already mentioned, if you try to eat two Devil Fruits, you'll die....So keep that in mind.

Types of Fruits: There are three types of Fruits; Paramecia, Zoan, and Logia.

Paramecia
These alter the user's body in some way or give the user a special ability which does not fit into the other categories....An example would be Luffy's abilities. These are the most common type. Of course, being the most common does not make them the weakest. It's all a matter of personal taste. "Whatever powers a Paramecia Fruit gives, it is based on the concept of either a characteristic like slipperiness or slowness, or an object like a bomb or a door."

Zoan
Zoan allow the transformation into an animal. This fruit provides three forms: the user's natural form, a full animal form, and a hybrid form, combining both the original and animal form. How could I best describe it...? It would be like Arrancar. Before they were Arrancarized, they had a "natural" Hollow form. When they were Arrancarized, they enter what could be described as a "Hybrid" form. When they release, that'd be their "Full Animal" form.

Logia
"Logia is the rarest of the three groups of Devil Fruit. They could be described as elemental fruits. Logia users cannot control their elements if the element is in another state of matter (i.e., Aokiji can control ice but not water). However, they may manipulate their surroundings into generating their element. Examples are Aokiji freezing an ocean, Crocodile turning a field of grass into a mini-desert, and Enel turning a cloud into a thunder cloud. These Devil Fruits are also not limited to the classical elements: fire, water, earth and air but includes physical and non-physical states like smoke, sand and darkness."

He asked when, exactly, can they be obtained for them though?

I want to ask how exactly you RP.