Prolific uses the maximum time to time-out submissions who are inactive (i.e. left the study). As a result, this cannot be too close to the estimated completion time to avoid unfairly excluding participants who take slightly longer to complete than your estimate.
If you need to set an absolute maximum completion time in your study, you should programme this within your survey software and make sure you clearly communicate this to your participants. If you do this, you can then reject submissions which go over this time or just remove them from your analysis.
The calculation for the minimum value of the maximum time allowed is as follows (where X is your estimated completion time):
Minimum Maximum Allowed Time = 2 + X + (2*sqrt(X))