Some times it feels like forever finding all 12. Other times, it's pretty quick.
I generally do my portal farming first and then when I'm tired of seeing Bloodhammer's face, I'll switch to running dense maps (Shuyal, Jurn, WT4) to finish finding any remaining needed.

But yes, there are times where I clear clear out WT4 and find just one infected. The rate of them does seem to be a bit low, considering just how fast the other daily gets completed.
It might be intended to prevent some people from making 50 lvl 10 toons to complete the 15 daily quests and pooling the 20k achievement gold to their main, along with all the gold each one would get from doing each set of dailies.