I just opened the .vmf file for dustbowl that valve supplies, in hammer. The logic_auto entity sets blu's respawn time to 2 on trigger "MultiNewMap" (when the map is loaded). I could not find *any* other entities that changed spawn times in the map (also, no spawn times were set in the map for red, meaning it would get set to the game's default value).
I also downloaded entspy and gcfscape, extracted cp_dustbowl.bsp from tf2's .ncf file, and found that the file in-game is the same as the vmf. The logic_auto entity has "tf_gamerules,setblueteamrespawnwavetime,2,0,-1" as a trigger on map start (set blu's spawn time to 2). There is no alternate "server" map (I have the dedicated server installed as well), so this is what the spawn time is supposed to be for blu.
The reason why the spawn times are rediculous is because 20 seconds is not the default time for blu.
This is precisely what the official tf2 wiki says, as well. The actual respawnwavetime for blu should be 2, and for red should be 10 (meaning max spawn times are 4 and 20 seconds, respectively).