You cannot and need not configure any such limit for Get Customer Input block. There is a max 8 second hard limit on Invoke AWS Lambda function block but it is not enforced when the lambda is invoked by Lex bot selected in Get Customer Input block.

Based on my testing, it appears that the Lambda invocation by Lex in the Get Customer Input block has a time limit that is greater than 25 seconds, though the exact time limit is not explicitly documented by AWS. So, feel free to leverage this block without worrying about the 8-second constraint.