The basics are weeks, not years: intervals, keys and common chords are learnable in 1-3 months of short daily sessions.
The beginnings are easier than their reputation. Notes, intervals, major and minor, and how chords fit a key: a motivated beginner covers that in a few weeks of short sessions.
What takes longer is making it automatic, which is a training problem, not an understanding problem. Ten minutes a day beats a heroic weekend.
A realistic timeline: basic fluency (reading simple notation, naming intervals, knowing the chords of a key) in 1-3 months of daily practice; comfortable intermediate theory (seventh chords, modes, roman-numeral analysis) inside a year; advanced topics indefinitely, like any deep field.
Theory feels hard for two fixable reasons: it is usually taught symbols-first instead of sounds-first, and it front-loads jargon. Attaching every concept to a sound you can recognize, then naming it, flips the difficulty.
Ear skills follow the same curve: most learners hear clear progress in interval and chord recognition within 2 to 4 weeks of daily 10-minute sessions.
The skill decomposes into knowledge (definitions, learnable in days), recognition (hearing the concepts, weeks to months of spaced practice), and application (using them while playing or writing, which rides on your instrument skills).
Recognition responds to the same training variables as physical skills: frequency beats duration, immediate feedback beats delayed, and difficulty pinned near 70 percent success maximizes learning rate, which is what an adaptive trainer automates.
Reading notation specifically: expect note-by-note decoding for the first weeks and genuine sight-reading of easy material in 3-6 months. It is a reps game; nobody reads fluently from explanations.