Avs

By now you've all seen this chart:


I'm sure this chart is accurate to an extent, but by no means does it actually tell you when X is spawning. But any data collected will also be an approximate because its really hard to write down the time (when you know something has spawned) while shooting things.

H = Horde, T = Tank

As far as I can tell, there are two constants for events which are marked by * below. Times are all listed in Minute:Seconds. Also note, there is a slight variation of when spawns occur by something like +/- 10 seconds. This is adjusted by the Lull (or so I am led to believe). When two times are listed, it shows an approximate range (multiple times recorded for the same type of spawn). Also, since its almost impossible to tell whether there are two tanks spawned (the music loops twice in the beginning even for single tanks sometimes and doesnt restart if a second tank spawns), I could not pinpoint the times (and use Valve's chart to estimate). Note that data becomes obscure after 14:00.

0:00 - H
0:40 - T*
1:00 - H
1:55/2:00 - TH
2:50/3:00 - TH
4:30 - TH (upwards of 4 special infected)
5:00 - H
5:30 - TT (two tanks)
6:30 - H
7:00 - T
7:40 - T
8:00 - H (upwards of 6 special infected)
8:20 - T
9:20 - H
10:00 - TH*
11:20 - TTH
12:00 - TTH
12:30 - H
13:00 - TTH
14:00 - TTH

At 18:00 - Panic Events are at 10 second intervals (infinite horde)
At 20:00 - Tanks spawn at an average of 10 seconds apart or something, or a lot heh.

Again by no means is this supposed to be accurate. But hopefully it will help you plan your strategies against where to be at what times for your survival games!

Avs

It is important to note that when the tank spawns post 4:00, occasionally it will not immediately make its way to the survivors. Generally this means it may show up either shortly before, or after, the next tank spawn creating a double tank scenario. True double tank scenarios do not occur until later (where Tanks will simultaneously attack at the same time). Though these all change defendant on:

1. Position of the players
2. The map layout itself
3. Your clearing speed (as this impacts lulls and stress level).

Having a good sense of natural lull times and how well your team is doing will allow you to extend lulls or initiate them before the actual "lull timer" begins. This is a necessary tactic/skill beyond the 12 minute mark if your strategy relies on ammo refills or item collecting which are generally necessary to down multiple tanks with multiple panic events and constant special infected attacks.