When - what date or what blockheight - would the tail emission kick in ?
It is hard to know the exact date/block because it depends on penalties. Each block's reward is a function of the
actual number of coins created so far.
It will kick in whenever the reward according to the formula would be less than 0.3 XMR (approx 0.9% annually), which is expected some time around year 8.
Edit: I guess what I'm asking is, what is the actual formula for the tail emission, the resultant inflation year over year and why was that amount chosen?