Discussed at MWLUG and although some numbers were stated, my own background/experience provided the simplest and most agreeable answer.
Hours that is. If you are stuck on a problem in your server, environment, car, whatever, don't spend more than 2 hours on it trying and retrying the same things.
Walk away. Forget about it. Get something to eat, go have a drink, just drop it. sleep on it.
It will come to you usually within an hour.
The subconscious is smarter than we think and is always working on what causes us problems.
Caution: Some people this will not help because they don't have the proper knowledge, but presuming you do, this should work for you.