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.
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.