Officially it is 270 seconds as per changes on 07.02.2023.
I play on Yonabra, today i've seen that in the most of the times that i have killed the fiendish creature a new creature appeared. I guess that there is some kind of timer (for example 5 minutes), so if you spend more than 5 minutes to find and kill this creature a new will appear immediately, but if you spend less than this time you will need to wait some time to a new spawn.
Official Replies from CM's
1. It takes five minutes for a fiendish monster to appear; while it's theoretically possible that they happen to be in the same area, it's rather unlikely as only a handful of them are present at the same time in the game world.1
2. Up to 3 fiendish monsters can be present at the same time on a game world. 2
1. https://www.tibia.com/forum/?action=thread&postid=39252863#post39252863
2. https://www.tibia.com/forum/?action=thread&postid=39252950#post39252950
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