I'd suggest the "Must bed-in the skill" quirk, applying at least in non-catgirls.
This means that you need at least two months practicing using the skill on a 5-days a week basis, for at least 15mins per day. This needs to be a continuous period in the first year after you get the skill, preferably the first six months. You can only be doing this with at most three skillsofts at once. Unless you do this then the skillsoft will 'fade', you'll find holes in it, and after two years at most there will be only fragments left. Fading starts after six months; the practice stops fading, and integrates the skill so its one of your own.
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"It is the business of the future to be dangerous" - Hawkwind
This means that you need at least two months practicing using the skill on a 5-days a week basis, for at least 15mins per day. This needs to be a continuous period in the first year after you get the skill, preferably the first six months. You can only be doing this with at most three skillsofts at once. Unless you do this then the skillsoft will 'fade', you'll find holes in it, and after two years at most there will be only fragments left. Fading starts after six months; the practice stops fading, and integrates the skill so its one of your own.
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"It is the business of the future to be dangerous" - Hawkwind