How long to self study Olympiads

Just for Chemistry, Biology, and physics. For a student who has taken an honours course in all subjects but not yet AP. I Hv in interest in all of them and I did basic GCSE level ( SAT Subject II level but for england) in all the subjects but want to continue in Olympiad. How long would iit take me to self study all the theory, not practicals for each olympiad.