The main thing to notice in this table is that intervals between spawns, when measured in real time, were almost always 304 or 305 seconds. (Those numbers are blue.) There was only one exception, between the fourth and fith spawns, when the interval was twice that length. In contrast, the same intervals when measured in game time were irregular, showing that game time doesn't run at a constant speed. The intervals between deaths, no matter how we measure them, were irregular.
Since real times in this table are only accurate to within a second, we can conclude from this that Prugor's timer length is 304.5 seconds of real time, and that his timer isn't affected by his deaths.
The reason the interval between the fourth and fifth spawns is twice that length is that halfway between them, the timer fired and the server decided not to spawn Prugor. That was the only time the dice roll failed.
Now we know one of the two numbers that determine the characteristics of Prugor's spawn: his timer length. It's 304.5 real seconds. Can we also figure out the second number, his probability? Yes, we can. We count the total number of spawns except for the first: 8. Then we count the number of elapsed timer periods: 9. We divide the first number by the second to get .89 and multiply by 100 to express it as a percentage: 89%. That's our estimate of Prugor's probability.
Most likely this estimate isn't very accurate because we used a small number of timer periods to calculate it. If we had recorded 1000 timer periods instead of only 9, the estimate would almost certainly be closer to the actual number used by the server.
How Long Does He Take To Pop?
Now that timers have been explained, we can tackle the question we began with: "How long does a particular mob take to respawn?"
Sometimes this question has a useful answer. With a mob like Prugor, whose probability is close to 100%, the question makes sense. He usually spawns every five minutes.
But with many rare mobs, the question doesn't have a useful answer. To see why, let's consider the typical rare mob discussed by Mythic in the Grab Bag article referenced in the sidebar.
According to Mythic, that typical mob has a timer length of one hour and a probability of 10 percent. With such a low probability, the mob's spawn intervals will be extremely irregular.
To get a better sense of this, I wrote a simulation program that calculates the intervals between spawns. (The source code is provided below.) Here's some sample output. Each number of hours is the elapsed time between one spawn and the next:

1. 
12 

hours 

2. 
1 

hour 

3. 
7 

hours 

4. 
1 

hour 

5. 
11 

hours 

6. 
5 

hours 

7. 
2 

hours 

8. 
11 

hours 

9. 
34 

hours 

10. 
1 

hour 
If you had been camping this mob, sometimes you would have waited only 60 minutes and other times more than a day and a half.
Now suppose somebody asks you, "What's this mob's spawn time?"
One way to answer would be, "It varies between an hour and 34 hours," but that's not much help.
You could also answer by giving the average spawn time, but there are two problems with this:
1. It's almost impossible to figure out the average spawn time of a rare mob in the game.
2. Even if you knew the average time, it wouldn't be helpful. We can see this by calculating the average time for our hypothetical mob with the formula length / probability. The average time is ten hours. But look at the numbers in the table. The actual spawn times were almost never close to ten hours. 