Firstly you don't 'build' collateral arteries, you are born with them. What happens in many people is that then 'open' when required. It is believed they are opened due to pressure differentials across the coronary arteries. This means they would open gradually as a blockage gradually grows. It also means that they could instantly open when a Heart attack occurs. So there is no set time involved, or average. This is how the therapy of EECP works. It tricks the collaterals into opening by affecting the pressure in the coronary arteries, using cuffs to restrict flow into the legs just at the right time.