The spawn time of fiendish creatures is currently 2 minutes and 15 seconds.
The time at which the next fiendish creature spawns is related to the time at which the previous one spawned, and has nothing to do with the time at which one was killed. So, as long as there are fewer than 4 fiendish creatures alive at any one time, they will keep spawning at 2 minute 15 second intervals, regardless of the time at which they are killed. If there are 4 fiendish creatures alive (the maximum number) when another one is due to spawn, then it is skipped, and the next chance is another 2 minutes and 15 seconds in the future.
This means you can predict the spawn times by just setting a clock to trigger every 2 minutes and 15 seconds.
This breaks down when a fiendish creature despawns. When that happens, another fiendish creature will immediately spawn to replace it. A fiendish creatures last for 60 minutes, but 2 minutes and 15 seconds does not evenly divide 60 minutes. Let's imagine a fiendish creature spawned at 11:00:00 exactly, but it is inaccessible and does not get killed. Assuming other fiendish creatures are getting killed, the respawn clock will progress at 2:15 intervals, from 11:00:00 to 11:02:15 to 11:04:30 and so on, all the way until 11:58:30. The next fiend after this would be due to spawn at 12:00:45, but it actually spawns immediately at 12:00:00 when the inaccessible one despawned. This results in the fiendish spawn clock shifting backwards by 45 seconds whenever a despawn happens.