not to sure on the spawn time. I usually do a /time and look at the game time.. usually during the morning is when they pop and they last until late afternoon.. if they're still alive they'll just dissapper. I'm just finishing up my newbie armor quests for craps and giggles.. It only took me 54 levels.
I was attempting to do my clerics greaves armor quest, and i got one of the tergites to drop. But there was so few of the large yellowjackets popping and I was gettin frustrated, that i actually made a ranger and have been lvl'in him up in an attempt to find these little boogers.
He's fixin to be lvl 8 and i've only tracked 2 largeyellow jackets in that time. These guys seem pretty rare.