To be honest, I think your question cannot be answered properly in its current format. There are too many loosely defined elements in your question.
For example, when you say 'learn kanji', what do you mean? Are you talking about a controlled experiment in which participants are given 20 kanji to learn and then have their retention rates tested intermittently? Or do you mean 'learn ALL the kanji required for basic literacy (2136 characters)'? Or do you want to include the Jinmeiyo kanji list too (another 800 characters)? And when you say 'learn kanji', do you mean that students should be completely competent in both receptive and productive skills, or just receptive skills? For example, someone could read a kanji accurately but be unable to reproduce it themselves. Or someone might be able to read a character in context without a solid knowledge of individual character readings. Some students learn how to write kanji before they can read them. Speaking of readings, how many readings should a learner know? Just the standard KUN and ON readings? Or exceptional readings and low-frequency readings too ? Or is it just a case of 'know everything about kanji'??
You see, learning kanji is an extremely complex task which involves a range of cognitive and metacognitive skills. You are asking general technical questions about the functions of memory without providing the necessary basis on which the question can be answered. Asking "how quickly can you learn kanji" is like asking "how quickly can you learn history?" or "how quickly can you understand art?" There are a million answers because the question is improperly formulated.
My advice is to re-think your question to get at what you really want to know. Are you trying to learn kanji and wondering how long it might take to achieve basic literacy? Or are you more interested in the cognitive demands that kanji places on short term and long term memory? Or something else? I'm sure there is a lot of academic literature to help you with what you are looking for, but I think we need a more detailed explanation of what you really want to know first.