When you have $15-20 billion/month in subsidizing commitments, internal and very proximal security problems and you remain a profligate consumer of imported goods, cash reserves dwindle quickly.
800 divided 20 is 40 months
Since oil price is only half and not zero, let us call it at least five years.
OPEC will crack if these prices are sustained. But it won’t be Saudi Arabia driving the changes from being unsustainable.