The chip Data Sheet usually gives these figures. They're usually very fast AS LONG AS you don't have to erase a sector first.
So make sure you have a pre-erased block, or a free area (at the end of the data in the current flash block) to write to.
So how much data do you have to write? Multiply that by the write-time.
To that you'll have to add the delay from power disconnect to when your hardware recognises the fall. You'll have the raw input DC connected to an ADC input through a divider. There's a filter (time constant) in there. Then you'll be polling the ADC at a specific rate. If you're polling at (say) 10ms, then the worst case will be the code sees the fall 10ms after it has fallen.
You might use external hardware to see the voltage fall and then trigger an interrupt, but an ADC is usually easier. If you do use external hardware, make sure it handles a "slow brownout" properly.
What do you do if the voltage drops and then recovers without going low enough to reset? Watch out for problems like this.
Tom
