The rate of crack propagation is not a constant for a given material. For a cracked material under constant load, the upper limit of crack speed is about 30 percent of the speed of longitudinal waves. So for ice, the maximum crack speed should be about 1100 m/s. The speed depends on length of the crack. It will start slowly and as the crack lengthens the speed will quickly approach this upper limit.
Most likely, if ice is cracking under your weight, the net load acting to open the crack will decrease rather than remain constant as the crack grows. This will slow or stop crack propagation, but the exact amount depends on the situation.