Knowing how long isn't definitive. The more aeration, the better. In my own pool when I just turn up the returns and put the pump on high, it takes about half a day to go from 7.0 to 7.2 or so at a lower TA of around 120 ppm, but every pool is different. Since your TA is higher, I'd figure it'll take a few hours to get to 7.2 and another half a day to get to 7.4 -- just try it out for a day to see. The process will go faster if you add acid when you see the pH get to 7.2. Basically, the carbon dioxide outgassing is faster at lower pH and slows down as the pH rises.