Maximum Execution Time
From WHMCS Documentation
By default, WHMCS enforces a max_execution_time limit of 300 seconds for general requests.
If this limit is too little for a given task or operation, you may encounter an error message that looks something like the following
PHP Fatal error: Maximum execution time of 300 seconds exceeded in /path/to/file.php
Increasing the max_execution_time
If you wish to override the max execution time limit and increase it to a higher value, you can do so by adding the following line to your WHMCS configuration.php file:
$overidephptimelimit = 500;