There have been studies of repetitive injury via more minor bumps (including heading the ball in soccer) that aren't viewed as injuries in the moment. While they do show a correlation that probably demonstrates causality, there's just not enough info yet on what the healing time is. We really know fairly little about how the brain heals - not that long ago, it was largely believed that it didn't do much of that. So, we just don't know what those passing bumps in the head during groundwork, jostling of the head when taking a bad fall from a throw, and light hits during moderate sparring will add up to.while that may be true. My guess a minor bump this week, a minor bump next week, another minor bump 2 weeks later wouldn't qualify as "enough healing time." If we hit a bruise with the same frequency, it wouldn't heal properly either. The bigger the bruise the more damaging the repeated impacts would be. For soft tissue like the brain I would think this would be even more so. From what they do understand about CTE, it's the repetitiveness that causes the damage.
People who have head injuries from accidents don't display similar CTE damage as those who have had repetitive impacts.
This is the healing time for traumatic brain injuries
"The person continues to improve between six months and two years after injury, but this varies for different people and may not happen as fast as the first six months. Improvements slow down substantially after two years but may still occur many years after injury."
Not sure if there is a study for minor bumps.