The key to learning - regardless of subject - is really repetition and association. Time is mostly incidental. Everybody has different learning styles, but it all essentially boils down to either repeating something over and over - which could be just reading and re-reading, saying it out loud, or simple practice - or linking the new knowledge to existing knowledge. This doesn't even get into the difference between learning and retrieval.

Phrases like "some level of competence" or "an acceptable level" are pretty abstract to begin with, so I always chuckle a bit when hearing statements including them.