In Dead Island, orange-rarity items spawn in loot drops (e.g., weapon chest) at a high, if not 100% chance (outside of vendors) after a certain amount of time. This information was largely propagated by DeathMule across fora. DeathMule claims that the interval between this guaranteed-orange-weapon-spawn-chance (owsc) is five hours. I feel that this claim draws an arbitrarily high time beyond and rather than the true value. Is it true that it takes exactly five hours or does it take a shorter length of time?
In my experience, there is no set period of time before the game provides you with an orange-rarity weapon. Across multiple playthroughs I can attest that while the weapon distribution seems random, it generally orients to your character level. At higher character levels your character is more likely to find higher quality weapons in searched containers.
Other than the post you've linked I have not seen any other indicator that weapons are distributed on a timed schedule. If it were true, few players would find more than five orange-tier weapons in a typical playthrough - far less than I've actually collected on playthroughs of my own.
When your character approaches experience levels where they can start regularly finding orange weapons I would claim to find a new one around every 45 to 75 minutes of playtime - a far shorter timeframe than DeathMule states. Regardless, it seems random as I have found two orange-rarity weapons within a ten minute period on more than one occasion.
If you are in search of powerful weapons try making use of skull mods to receive potent developer blueprints, instead.