If one alters the weld time from the desired range and exceeds the extreme either too long or short you risk surface expulsion, electrode mushrooming, and excessive indentation. If weld times are short you risk small nuggets and low weld strength or no welds.
Long weld times with proper currents lead to excessive heating. When this happens one would expect to see expulsion probably from the electrode interface which has gotten excessively hot. This same heat leads to accelerated electrode wear in the form of mushrooming. The part develops a large molten area and the electrode will indent the part more than desired.
Short weld times risk the opposite. Not enough time for an adequate nugget to form and the weld is either weak or nonexistent.