Well within a fraction of a cycle/second. One of the big reasons is that a generator's mechanical speed is directly tied to the electrical frequency. This includes the prime-movers that actually drive the generators. Many of these large machines don't react well to operating far from their design speed, which can cause huge issues.

.01 cycle was my memory of a threshold from some years ago.

Part of the problem is lower frequencies don't meet as much impedence in inductors (e.g., transformers), and another part is that interconnects can have control system stability problems keeping them synced.