Great question. Could not find the answer but I did find this:
The Nasdaq-100 is officially rebalanced once each year, on the 3rd Friday in December. Not surprisingly, that’s also when most new additions take place (Chart 3). However, index changes can happen during the year if a company is acquired or if securities fail the ‘Continued Eligibility Criteria,’ which includes becoming too small during the year. Historically around 10% of the companies rotate out of the Nasdaq-100 each year (although usually, these are the smaller companies, so the impact on turnover is lower).