you learn kanji all your life, and if you stop, you will start forgetting them. There is no time limit for it either, as everyone has different ways and abilities of "inhaling" them. Also, learning single kanji makes no sense, you learn them in phrases, such as 漢字, otherwise it'll take you more time and will be frustrating. There are flashcards with kanji online:
Speedanki: Kanji Flash Cards with JLPT Vocabulary
It takes time for us foreigners to learn them, as Asians employ (or learn to employ) different part of the brain for learning kanji from a very young age, and they understand them and comprehand them in a different way.
Finally, I suggest you learn the correct stroke order first, and read about kanji construction. It will benefit your further studies. Undertsanding kanji parts and radicals is crucial, in my opinion, in successful studies of Chinese characters.