We know that the limit is 6 minute / execution. Fine!
My code runs in 1 to max. 4 minutes, once every 30 minutes.
Clearly, even the max 4 minutes is less than the 6 minute limit per execution. Yet I get the following error message sometimes:
Service using too much computer time for one day
So, what is the TOTAL DAILY limit for all executions combined?
According to the official documentation:
Service using too much computer time for one day. This indicates that the script exceeded the total allowable execution time for one day. It most commonly occurs for scripts that run on a trigger, which have a lower daily limit than scripts executed manually.
Again on the same page, you can see that there is the following limitation on the total time you can run a script via triggers:
Therefore, if you have a consumer account you can only run a script via trigger for 90 minutes.