I know you hate it when I do this, but I don't have a lot of spare time and Ralph Losey just did another excellent treatise on the "SUNY" case, which sheds some light on when to issue a litigation hold.
I know you want to know about this because my stats tell me that over a year after I first posted it, my sample of a litigation hold letter – which you see over on the left sidebar – is still the most popular item on this blog.
So, I highly recommend you take a look; and pay close attention to the contrast in privileges. We need all the guidance we can get.