0.3 % would correspond to 3 mm difference in length of the pendulum.
After an hour, the difference between real and measured time would already be 10.9 s, and over an entire day, it would accumulate to 261.3 s, way too much for useful long term measurements.
Yet, it is an useful approximation for qualitative measurements, e.g. when Galileo Galilei did his fall experiments, he might have used a prendulum instead of his pulse for measuring.