小中大【回复】
It depends upon the types of your samples.
For example, if your samples are very brittle, there is no necessity for a long time, since it might even tend to be fractured ASAP.
In contrast, if your samples are of high tougheness(elastomer, e.g), more time is required, since it hardly be totally broken.
In sum, you should unify the time in order to eliminate the experimental errors.
