Well, it's not the most specific of time frames, but we have a target for that Shield Tablet update to Android Lollipop. Nvidia has posted a new "sneak peek" video that shows its tablet running Android 5.0, and the video says we can expect the update this month. Here's incredibly well-spoken guy with a small amount of info:
If Nvidia delivers on this schedule, the Shield Tablet should be among the first devices to receive an update to Lollipop.
I'm still waiting for the update for my Nexus 7, and according to widespread web reports, I should already have it, but not really, since maybe it's postponed, but Google isn't saying, and hopefully we'll find out more soon.