Breaking your limiter is instantaneous when one becomes an incubus. Now, depending on the circumstances, someone might take years to become an incubus, while some might become an incubus within a single night, this depends on certain things like the kind of monster girl they got with, or in the case of men who became incubi before getting a mamono wife, simply from the ambient mamono mana in the area they spent their time in. Breaking one's limiter doesn't mean gaining immediate power however, one still has to go through training and learning, and things like predisposition and willpower can be very important here. That training might take years. There are ways to speed it up, but it would be difficult. An incubus with enough willpower and determination can go from being scrawny to matching his Lizardman or dragon wife in battle as an equal, but there's no way to avoid the training it also takes to get there, and depending on one's interpretation, you could argue that such a wife had chosen that man because she saw the potential in him.
I already see some issues with the MC being an incubus of a harem, wherein one girl is all it takes for him to pass out for the night.
Being an incubus is important as it relates to the kinds of monster girl wives one has, and so our MC who has a harem, should change based on each one of his wives. I expect there would be some passive effects based on that, but don't know if that could realistically be implemented, since it might simply be too much. If our MC is going to have a very large and varied harem of monster girl wives, that in itself means he's a very special snowflake, even moreso that one of them is a Lilim. Not sure if KC is planning to make them an important figure in the setting as a whole, or keep them on the background.