If the global hashrate were to suddenly drop, there'd a a huge uptick in the value of a single miner - e.g. my 400MH/s would suddenly be able to generate blocks quite quickly and I'd earn several hundred BTC in a fraction of the time it takes now.
Well - my question was how quickly this would happen, because if you think that this would happen immediately - then you are mistaken.
It is simple math, difficulty adjusts every 2016 blocks, and it adjusts by Expected_time_to_complete/Actual_Time_To_Complete * Difficulty, with a max of 4x up/down.
The "time to adjust" depends on when the drop off occurred, how big a drop off, etc.
For example, say for block 1-1008 hashing went along at 1 block/10min. Then at block 1008-2016, half the hashrate disappeared, giving 1block/20min. It would take 3 weeks to complete 2016 blocks. 2weeks/3weeks = 2/3rds of difficulty, even though hashrate dropped by half, meaning another 2.25 or so weeks before a drop to proper difficulty (but at 1block/15min). Had half dropped at block 1, it would have been 1/2difficulty after 4 weeks of 1block/20min.
And so on.