Saga of Jaraah
Besides the proficiency slots characters have from intelligence, class, and level, characters can take time off from adventuring to learn additional non-weapon proficiencies. For each slot required for basic proficiency, the character must dedicate 3 months of study. For example, languages are 1 slot, so characters must study the language for 3 months to learn it. Weaponsmithing is 3 slots, so characters must study weaponsmithing for 9 months to learn it. Bards gain musical instruments for 1/2 slot, so they only need 1.5 months to learn new instruments. Study time must be distinct from adventuring, traveling, convalescing, paying penance, etc. It must also be in a reasonably solid chunk of time.
Characters wishing to improve existing proficiencies through study can do so, but the time spent to improve increases for each slot beyond basic proficiency. For example, Sorkin wishes to learn weaponsmithing. He spends 9 months learning the basic craft. He wants to get really good, so he spends 6 months for an additional slot beyond the first 3 to give him a +1 bonus to his check. He still wants to be better, so he now spends 9 months for a second additional slot to give him a cumulative +2 bonus to his check.
Slots gained through level advancement do not require any extra time set aside to learn. These are assumed to be learned by ‘squeezing in’ study here and there while adventuring which is why extra slots can only be earned by taking time off. Dedicating a lot of extra slots to non-weapon proficiencies allows characters to perform truly exceptional feats. Having basic proficiency in weaponsmithing allows Sorkin to create perfectly functional weapons. Having a bunch of extra slots allows him to create weapons that are works of art in their own right capable of receiving magical enchantment or even having bonus to hit or damage through craftsmanship alone. Similarly, having basic proficiency in an instrument allows competent and occasionally inspired performances, but lots of extra slots allow routine virtuoso delivery and dominance in any kind of competition against other trained musicians.