Ignoring the economic reasons as to why this may or may not happen. If for some reason the network hashrate suddenly dropped by 30% immediately after a difficulty adjustment and remained at that level until the next difficulty adjustment, how long would each block be expected to take to mine until the next difficulty adjustment.
1 Answer
You're trying to do 100% of the work with 70% of the workforce.
The expected block interval is to the target interval of 10 minutes as the total work is to the workforce:
Texp = 10 minutes × (100% ÷ 70%) = 14.3 minutes
The difficulty reset would get extended by the same factor:
Dreset = 14 days × 100 ÷ 70 = 20 days
The expected block interval would be about 14.3 minutes for twenty days.
