Depends on what you mean by learn... And what other relevant background one has outside of CS - programming/math/stats/systems/engineering. Assuming one has a f

Depends on what you mean by learn... And what other relevant background one has outside of CS - programming/math/stats/systems/engineering. Assuming one has a few years of that, and one want to get to the level of being able to apply this to a problem that comes up in a realistic setting (making a product etc). Maybe extreme dedication over many months, or a few years at 20 hours a week? This is assuming that one either is very good at studying alone, or have access to people one can ask alone the way. If one has no relevant background, then I would 5+ years of full time study.