On big attractions, wait times will be less than half of what it is for the regular lines. On many rides, the single rider lines could be even shorter than the express line (but don't bank on this).
If Ripsaw Falls, IOA's most popular water ride, has a wait time of 60 minutes - which is a conservative estimate between midday and 5pm on a hot day - then Express could well get you on in between 20-30 minutes.