Honestly, it's really dependant on the specific currency and what news/events are upcoming and whether they're on, above or below target like GDP etc. or whether it's a positive impact on the countries economy or not. There's a large number of factors that can strengthen or weaken a currency tbh that you need to know so I'd recommend doing your research on these and what can causes crash. Another suggestion is constantly keeping an eye on the financial sectors of newspapers.
There's no easy answer really but in short, no it's not normally quick from my experience but it can be.
The best thing to do is to keep an eye on what's coming up which will be relevant or influential to the currency.