What would the expected block mining time be if 30% of the total network hashrate disappeared suddenly?
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.