There's a relatively new concept for dyslipidemia regarding how long the LDL cholesterol has been elevated. If you take your age times your LDL cholesterol and that value is over 5,000, you are at increased risk of atherosclerosis. Somebody's developed the calculator that will show you the benefits of interventions for lowering lipoproteins. You can also enter historical information about previous LDL cholesterol to get a better measurement.
This is akin to pack years of cigarettes. If you smoke one pack of cigarettes one time, that's probably not an issue, but if you smoke one pack of cigarettes daily for 40 years, you'll probably have COPD and heart disease.