By now you've all seen this chart:

[media]http://www.l4d.com/blog/images/posts/012/spawn-graph-thumb.jpg[/media]

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!