To determine how often a Flip Flop will go to an undefined state, plug the data into one of these equations to calculate the MTBF.

Definitions

MTBF: Mean Time Between Failure
FD: Data Frequency
FC: Clock Frequency
T_P: Flip Flop Propagation Delay
tr: Resolve Time
dt: Delay Time between Clocks [symbole delta t]
tsu: Device Setup Time
g: Flip FLop Resolution Time [symbol gama]

These are device dependent. Resolve time (among others) has to be looked up, via the data sheet (if it's provided).
As a rule: The faster the flip flop used, the better the MTBF for a given circuit.
The faster device families have lower Set-up and Hold times. This reduces the window of occurrence.