For high blood pressure to cause heart attack how long must it be high?

Impossible to answer: Believe it or not, htn, while a risk factor, is a relatively weak risk factor for heart attack. It's a big risk factor for stroke and congestive heart failure. (also kidney failure). Generally, risk factors must be present for decades to cause a heart attack, thus the "coronary prone age" is usually considered to be >55 for men and >65 for women (with many younger exceptions!).

Years: High blood pressure contributes to the formation of atherosclerotic plaques in coronary arteries. This process takes years to develop.