It can happen very quickly. I had this happen to me during one spring opening a few years ago and it took less than one week -- probably 3-4 days. My CYA went from around 30 ppm to close to zero, the pool turned cloudy, and I had a huge chlorine demand from ammonia and partial CYA products (probably biurate and allophanate) that came from bacterial biodegradation of CYA. A log of my chlorine usage to clear the pool is in this post. A technical explanation of the biodegradation of CYA is in this post.

Note that I had to add a LOT of chlorine and that it got consumed very quickly early on, so add it multiple times during the day measuring until the chlorine starts to hold. Once you get an FC reading that is mostly stable (i.e. not dropping to zero in hours), then you'll know that you've killed off the bacteria because they can't survive when FC is present. It's at that point that you can start to add some CYA to the water, but not too much (say, not more than 30 ppm) until you lose < 1 ppm FC overnight.