Atherosclerosis. Everything will be f9!

How long does it take to cholestrol to clog your arteries, i had high cholestrol since june which was very high, doctor told me to diet see for 3 momth yes there was improvement but not too much it was 166 now down from 181, now my cholestrol is definitely due to diet which i was on for like 15 months and lets say it would have took 1 month for cholestrol too rise so basically i have had very high cholestrol in my blood since 18 month, is 18 months enough to clog your arteries. And looking at rate i am loosing cholestrol i think it would take me another 6 months too lower it so 24 month of high ldl, how dangerous is this i have started taking plant sterols as supplement to lowere cholestrol after consulting a doctor and dietician, is 24 months of having cholestrol enough to damage arteries significantly or it is normal nothing to worry as i am 21 and if i continue to have normal cholestrol level rest of my life everything will be f9 regarding heart ofcourse, or it had already increased