Strictly speaking, the analysis provided by @BillJ seems correct, but grammar is mostly descriptive, so I'll describe my perspective.
The main noun in "A year's worth of learning" could be "year's", or more precisely the the omitted referent thereof, while "worth of" is an atypical adjective + preposition, "worth of learning" becoming an adjectival phrase. At least that's how I parse the sentence.
Compare that to "a ship('s storage) full of rice" - content would be the referent of "sack's". Likewise, a sack of rice and a year of learning are understood to mean the same, so I'd say sack or year are the head noun. I supposes that's what you mean by main noun. After all, the basic clause expressed here is "a ship is filled with rice". BillJ is still correct, because by omission of the referent, the adjective moves into the position of a noun. This is more apparent in other examples, because the adjective and noun form of worth don't differ. E.g. we could say a sack('s) filling of rice. That could be corrected to "a sack rice filling", sack becoming a quantifier, the common noun moving into the position of the noun, the verb moving into the participle position. In other languages, the preposition can be omitted even, so it's simply "a sack rice" (e.g. in German).
All that is motivated by the observation of a disagreement about countability (a type mismatch) from pairing an uncountable noun (the worth) with a countable noun (lessons, chocolates, houses). Clearly, "a dollar's worth of chocolate/chocolates" can't be both correct - with "learnings" it wouldn't even make sense, with "lessons" it would again be a type mismatch, whereas "a year of lesson" sounds wrong to begin with. But the length of my explanation hints at the simpler explanation being more likable (not to say likely). On the other hand, countability is a difficult topic, apparently, as the lexical aspect of worth implies countability to some degree. If a binary decision depends on gradual differences, we use fuzzy logic (how many grains are needed to count as sand?). In Linguistics, this is touched upon by Underspecification, i.e. with regards to the function of "worth" in the sentence.
As I said in a comment, this is marketing speech, so the rules of grammar don't strictly apply, anyway. The language might be imprecise on purpose, or simply because language is inherently subject to interpretation.