There doesn’t appear to be a way change the 5 mins value, at least as far as I can tell.
But Note: If you add the a project and spend less than 5 minutes on it(say 2 mins), and STOP the record. You will see the ACTUAL time you spend on the project saved in the Time Entry Record.(so 2 minutes, instead of 5.)

Also note: if the clock is still running on the project, in other words you are actively “working on” it, then the time entry record shows 5 minutes until 5 minutes is exceeded.
My best GUESS on the reasoning of this is if you want to manually change/add time, a 5 minute block of time is easier to click on than a 1 minute block is.