I suppose that making it a PrC is one way to make it alright... it's been discussed before, and I wasn't too sure about it. However, if people don't have a problem with a 20-level PrC, I guess it's alright.
How's this for a rough draft...
REQUIREMENTS
To qualify to become a learner, a character must fulfill all the following criteria.
Skills: Knowledge (arcana) 6 ranks, Knowledge (nature) 6 ranks.
Feats: Endurance.
Spells: Able to spontaneously cast at least 2nd-level arcane spells.
CLASS FEATURES
Spells per Day/Spells Known: At 1st level and every odd level thereafter (3rd, 5th, etc), the character gains new spells per day and spells known as if he had also gained a level in whatever arcane spellcasting class he belonged to before he added the prestige class. He does not, however, gain any other benefit a character of that class would have gained. This essentially means that he adds the level of learner to the level of whatever other arcane spellcasting class the character has, then determines spells per day and caster level accordingly.
If a character had more than one arcane spellcasting class before he became a learner, he must decide to which class he adds each level of learner for the purpose of determining spells per day.
Blue Spells: In addition to his arcane spells, the learner also gains access to spells with the Blue descriptor. These are special spells that are learned from observing or being subject to monster abilities (see Observation, below).
Unlike his standard spells, the learner gains access to blue spells based upon his Intelligence, rather than what ability his spells from previous classes are tied to.